Relatively Positioned Sprites

[pathunstrom]

Requires #666

Specifically intended as subsprites (#577), a RelativeSprite uses a separate attribute for its position. We'll call it relative_position. RelativeSprite.position becomes a property that returns the relative position in relation to the position and rotation of its parent.

For now, we'll leave RelativeSprite's own rotation alone, but if testing proves it necessary, rotation will need a similar split.

The basic idea:

return self.relative_position.rotate(self._parent.rotation) + self.parent.position

The goal is so that things like skeleton sprites (sprites who are made up of multiple sprites for animations), subordinate sprites (think r-types support ships), and particle systems don't need to worry about where they are in relation to the rest of their objects and can just move themselves, then get places appropriately by the renderer.