Implement layer draw order management

[shdwcat]

Currently YUI uses depth to control draw order and make sure the right elements draw on top of each other. But this makes it hard to implement features like shader and animation support, since it's extremely difficult to determine when a 'layer' is done drawing. Instead we should use a single GML layer to control draw order for things in the same 'YUI layer' (the base of a screen is one layer, windows and popups get their own layer, and users should be able to mark any element as the start of a new layer as well). Once draw order is managed via layers, we can use layer features for shaders and motion/animation/etc.