Subsprite Rendering

[pathunstrom]

Requires #665

Use the new render_walk to draw every sprite in the scene tree.

[AstraLuma]

Open questions:

• Are subsprites positioned relative to the parent or the scene?
• If they're positioned relative to the parent, does that frame of reference rotate with the parent?

[pathunstrom]

Initial pass: I'm only going to support global positioning. I don't want that to be the 'forever' decision.

I can imagine a pattern that allows us to support both fairly simply. I tend to consider relative positioning to be the "advanced" feature, so we could have a flag of some sort (Sprite.relative_positioning for example) that allows the renderer to switch to a relative mode for any given object.

As for that rotation, I feel like the answer should likely be yes, but that's another area where both answers might be correct.