Add a SpriteLayer class for handling sprites in a Scenelike.
Allow multiple layers to render non-forced if they promise not to overlap
(another optimization). Levels must not draw outside of game.area or
they'll fuck up the status bar. If you pass game.area to SpriteLayer, it
will do this for you!
Return true from Sprite.move if the move was successful.
Make Scenelike.render return a list of rects of areas on the screen that have changes. 1000 fps woo!
More gracefully handle not being able to use display.update.