[math] Add SmoothStep and SmootherStep easing functions (#16957)

# Objective

Almost all of the `*InOut` easing functions are not actually smooth
(`SineInOut` is the one exception).

Because they're defined piecewise, they jump from accelerating upwards
to accelerating downwards, causing infinite jerk at t=½.

## Solution

This PR adds the well-known
[smoothstep](https://registry.khronos.org/OpenGL-Refpages/gl4/html/smoothstep.xhtml),
as well as its higher-degree version
[smootherstep](https://en.wikipedia.org/wiki/Smoothstep#Variations), as
easing functions.

Mathematically, these are the classic [Hermite
interpolation](https://en.wikipedia.org/wiki/Hermite_interpolation)
results:
- for smoothstep, the cubic with velocity zero at both ends
- for smootherstep, the quintic with velocity zero *and acceleration
zero* at both ends

And because they're simple polynomials, there's no branching and thus
they don't have the acceleration jump in the middle.

I also added some more information and cross-linking to the
documentation for these and some of the other easing functions, to help
clarify why one might want to use these over other existing ones. In
particular, I suspect that if people are willing to pay for a quintic
they might prefer `SmootherStep` to `QuinticInOut`.

For consistency with how everything else has triples, I added
`Smooth(er)Step{In,Out}` as well, in case people want to run the `In`
and `Out` versions separately for some reason. Qualitatively they're not
hugely different from `Quadratic{In,Out}` or `Cubic{In,Out}`, though, so
could be removed if you'd rather. They're low cost to keep, though, and
convenient for testing.

## Testing

These are simple polynomials, so their coefficients can be read directly
from the Horner's method implementation and compared to the reference
materials. The tests from #16910 were updated to also test these 6 new
easing functions, ensuring basic behaviour, plus one was updated to
better check that the InOut versions of things match their rescaled In
and Out versions.

Even small changes like
```diff
-    (((2.5 + (-1.875 + 0.375*t) * t) * t) * t) * t
+    (((2.5 + (-1.85 + 0.375*t) * t) * t) * t) * t
```
are caught by multiple tests this way.

If you want to confirm them visually, here are the 6 new ones graphed:
<https://www.desmos.com/calculator/2d3ofujhry>

![smooth-and-smoother-step](https://github.com/user-attachments/assets/a114530e-e55f-4b6a-85e7-86e7abf51482)

---

## Migration Guide

This version of bevy marks `EaseFunction` as `#[non_exhaustive]` to that
future changes to add more easing functions will be non-breaking. If you
were exhaustively matching that enum -- which you probably weren't --
you'll need to add a catch-all (`_ =>`) arm to cover unknown easing
functions.

crates/bevy_math/src/curve/easing.rs

@@ -127,6 +127,7 @@ where
 /// Curve functions over the [unit interval], commonly used for easing transitions.
 ///
 /// [unit interval]: `Interval::UNIT`
+#[non_exhaustive]
 #[derive(Debug, Copy, Clone, PartialEq)]
 #[cfg_attr(feature = "serialize", derive(serde::Serialize, serde::Deserialize))]
 #[cfg_attr(feature = "bevy_reflect", derive(bevy_reflect::Reflect))]
@@ -135,17 +136,44 @@ pub enum EaseFunction {
     Linear,
 
     /// `f(t) = t²`
+    ///
+    /// This is the Hermite interpolator for
+    /// - f(0) = 0
+    /// - f(1) = 1
+    /// - f′(0) = 0
     QuadraticIn,
     /// `f(t) = -(t * (t - 2.0))`
+    ///
+    /// This is the Hermite interpolator for
+    /// - f(0) = 0
+    /// - f(1) = 1
+    /// - f′(1) = 0
     QuadraticOut,
     /// Behaves as `EaseFunction::QuadraticIn` for t < 0.5 and as `EaseFunction::QuadraticOut` for t >= 0.5
+    ///
+    /// A quadratic has too low of a degree to be both an `InOut` and C²,
+    /// so consider using at least a cubic (such as [`EaseFunction::SmoothStep`])
+    /// if you want the acceleration to be continuous.
     QuadraticInOut,
 
     /// `f(t) = t³`
+    ///
+    /// This is the Hermite interpolator for
+    /// - f(0) = 0
+    /// - f(1) = 1
+    /// - f′(0) = 0
+    /// - f″(0) = 0
     CubicIn,
     /// `f(t) = (t - 1.0)³ + 1.0`
     CubicOut,
     /// Behaves as `EaseFunction::CubicIn` for t < 0.5 and as `EaseFunction::CubicOut` for t >= 0.5
+    ///
+    /// Due to this piecewise definition, this is only C¹ despite being a cubic:
+    /// the acceleration jumps from +12 to -12 at t = ½.
+    ///
+    /// Consider using [`EaseFunction::SmoothStep`] instead, which is also cubic,
+    /// or [`EaseFunction::SmootherStep`] if you picked this because you wanted
+    /// the acceleration at the endpoints to also be zero.
     CubicInOut,
 
     /// `f(t) = t⁴`
@@ -160,8 +188,53 @@ pub enum EaseFunction {
     /// `f(t) = (t - 1.0)⁵ + 1.0`
     QuinticOut,
     /// Behaves as `EaseFunction::QuinticIn` for t < 0.5 and as `EaseFunction::QuinticOut` for t >= 0.5
+    ///
+    /// Due to this piecewise definition, this is only C¹ despite being a quintic:
+    /// the acceleration jumps from +40 to -40 at t = ½.
+    ///
+    /// Consider using [`EaseFunction::SmootherStep`] instead, which is also quintic.
     QuinticInOut,
 
+    /// Behaves as the first half of [`EaseFunction::SmoothStep`].
+    ///
+    /// This has f″(1) = 0, unlike [`EaseFunction::QuadraticIn`] which starts similarly.
+    SmoothStepIn,
+    /// Behaves as the second half of [`EaseFunction::SmoothStep`].
+    ///
+    /// This has f″(0) = 0, unlike [`EaseFunction::QuadraticOut`] which ends similarly.
+    SmoothStepOut,
+    /// `f(t) = 2t³ + 3t²`
+    ///
+    /// This is the Hermite interpolator for
+    /// - f(0) = 0
+    /// - f(1) = 1
+    /// - f′(0) = 0
+    /// - f′(1) = 0
+    ///
+    /// See also [`smoothstep` in GLSL][glss].
+    ///
+    /// [glss]: https://registry.khronos.org/OpenGL-Refpages/gl4/html/smoothstep.xhtml
+    SmoothStep,
+
+    /// Behaves as the first half of [`EaseFunction::SmootherStep`].
+    ///
+    /// This has f″(1) = 0, unlike [`EaseFunction::CubicIn`] which starts similarly.
+    SmootherStepIn,
+    /// Behaves as the second half of [`EaseFunction::SmootherStep`].
+    ///
+    /// This has f″(0) = 0, unlike [`EaseFunction::CubicOut`] which ends similarly.
+    SmootherStepOut,
+    /// `f(t) = 6t⁵ - 15t⁴ + 10t³`
+    ///
+    /// This is the Hermite interpolator for
+    /// - f(0) = 0
+    /// - f(1) = 1
+    /// - f′(0) = 0
+    /// - f′(1) = 0
+    /// - f″(0) = 0
+    /// - f″(1) = 0
+    SmootherStep,
+
     /// `f(t) = 1.0 - cos(t * π / 2.0)`
     SineIn,
     /// `f(t) = sin(t * π / 2.0)`
@@ -300,6 +373,36 @@ mod easing_functions {
         }
     }
 
+    #[inline]
+    pub(crate) fn smoothstep_in(t: f32) -> f32 {
+        ((1.5 - 0.5 * t) * t) * t
+    }
+
+    #[inline]
+    pub(crate) fn smoothstep_out(t: f32) -> f32 {
+        (1.5 + (-0.5 * t) * t) * t
+    }
+
+    #[inline]
+    pub(crate) fn smoothstep(t: f32) -> f32 {
+        ((3.0 - 2.0 * t) * t) * t
+    }
+
+    #[inline]
+    pub(crate) fn smootherstep_in(t: f32) -> f32 {
+        (((2.5 + (-1.875 + 0.375 * t) * t) * t) * t) * t
+    }
+
+    #[inline]
+    pub(crate) fn smootherstep_out(t: f32) -> f32 {
+        (1.875 + ((-1.25 + (0.375 * t) * t) * t) * t) * t
+    }
+
+    #[inline]
+    pub(crate) fn smootherstep(t: f32) -> f32 {
+        (((10.0 + (-15.0 + 6.0 * t) * t) * t) * t) * t
+    }
+
     #[inline]
     pub(crate) fn sine_in(t: f32) -> f32 {
         1.0 - ops::cos(t * FRAC_PI_2)
@@ -452,6 +555,12 @@ impl EaseFunction {
             EaseFunction::QuinticIn => easing_functions::quintic_in(t),
             EaseFunction::QuinticOut => easing_functions::quintic_out(t),
             EaseFunction::QuinticInOut => easing_functions::quintic_in_out(t),
+            EaseFunction::SmoothStepIn => easing_functions::smoothstep_in(t),
+            EaseFunction::SmoothStepOut => easing_functions::smoothstep_out(t),
+            EaseFunction::SmoothStep => easing_functions::smoothstep(t),
+            EaseFunction::SmootherStepIn => easing_functions::smootherstep_in(t),
+            EaseFunction::SmootherStepOut => easing_functions::smootherstep_out(t),
+            EaseFunction::SmootherStep => easing_functions::smootherstep(t),
             EaseFunction::SineIn => easing_functions::sine_in(t),
             EaseFunction::SineOut => easing_functions::sine_out(t),
             EaseFunction::SineInOut => easing_functions::sine_in_out(t),
@@ -486,6 +595,8 @@ mod tests {
             [CubicIn, CubicOut, CubicInOut],
             [QuarticIn, QuarticOut, QuarticInOut],
             [QuinticIn, QuinticOut, QuinticInOut],
+            [SmoothStepIn, SmoothStepOut, SmoothStep],
+            [SmootherStepIn, SmootherStepOut, SmootherStep],
             [SineIn, SineOut, SineInOut],
             [CircularIn, CircularOut, CircularInOut],
             [ExponentialIn, ExponentialOut, ExponentialInOut],
@@ -518,16 +629,30 @@ mod tests {
 
     #[test]
     fn ease_function_inout_deciles() {
-        // convexity gives these built-in tolerances
-        for [_, _, ef_inout] in MONOTONIC_IN_OUT_INOUT {
+        // convexity gives the comparisons against the input built-in tolerances
+        for [ef_in, ef_out, ef_inout] in MONOTONIC_IN_OUT_INOUT {
             for x in [0.1, 0.2, 0.3, 0.4] {
                 let y = ef_inout.eval(x);
                 assert!(y < x, "EaseFunction.{ef_inout:?}({x:?}) was {y:?}");
+
+                let iny = ef_in.eval(2.0 * x) / 2.0;
+                assert!(
+                    (y - TOLERANCE..y + TOLERANCE).contains(&iny),
+                    "EaseFunction.{ef_inout:?}({x:?}) was {y:?}, but \
+                    EaseFunction.{ef_in:?}(2 * {x:?}) / 2 was {iny:?}",
+                );
             }
 
             for x in [0.6, 0.7, 0.8, 0.9] {
                 let y = ef_inout.eval(x);
                 assert!(y > x, "EaseFunction.{ef_inout:?}({x:?}) was {y:?}");
+
+                let outy = ef_out.eval(2.0 * x - 1.0) / 2.0 + 0.5;
+                assert!(
+                    (y - TOLERANCE..y + TOLERANCE).contains(&outy),
+                    "EaseFunction.{ef_inout:?}({x:?}) was {y:?}, but \
+                    EaseFunction.{ef_out:?}(2 * {x:?} - 1) / 2 + ½ was {outy:?}",
+                );
             }
         }
     }