Add support for multi-shape sprites

[tmontes]

Facts

• There are currently two types of supported sprites: bitmap and vector based.
• Both backed by a single shape object.
• Bitmap shapes represent a single "image".
• Vector shapes represent a single closed polygon.

Idea

• Could we have a multi-shape backed sprite?
• It would allow for visually rich, multi-color vector based characters.
• It could support (no idea how!) animation by moving/rotating the individual shapes relatively to themselves or the sprite's anchor point.
• Feels interesting, fun, and powerful.

PS: Famous last words: how hard can this be?! :-)