Feature request: Add background parameter for `sbar()` when `type = "bar"`

[lsalinasm23]

Currently, the sbar() function in terra allows adding a white background (“halo”) only when type = "line" via the argument halo = TRUE.
However, when type = "bar", there is no way to add a background color behind the scale bar. This becomes an issue when the scale bar is drawn over complex basemaps (e.g., satellite or topographic images), where the divisions and labels become difficult to read.

It would be useful to include an argument such as:

bty = "o"          # draw box around the scale bar
box.col = "black"  # box border color
bg = "white"       # box background color (with support for transparency)

This would make it possible to create a readable and visually consistent scale bar, for example:

# Example: semitransparent white background for readability
sbar(15, 
c(6.3, 50),
type="bar",
below="km",
label=c(0,7.5,15), cex=.8,
bty = "o",
box.col = "black",
bg = rgb(1, 1, 1, alpha = 0.7)
)

This would improve the visibility of the scale bar on maps with dark or detailed backgrounds, and maintain consistency with how other base plotting functions (like legend()) handle background color.

Furthermore, implementing the same argument for the "line" type could replace and simplify the current halo parameter, providing a unified and more flexible way to control background color for both types of scale bars.

[rhijmans]

halo now also works for "bar" type --- box still to be done.

[lsalinasm23]

Hello @rhijmans,

Thank you very much for the implementation. I’ll be keeping an eye out for when the box support is ready