Add support for multiple maps on one page

[zack-krejci]

Requires significant restructuring. Must pass shader down to Layer somehow, maybe in draw stage? Or maybe it should request one?

[zack-krejci]

Too many significant engineering obstacles to implement this. Issue derives from WebGLBuffers not being able to share resources across Contexts, meaning we need a set of shaders for each map, which is nontrivial since layers can be created without specifying a map. Probably would still attempt it, but resource sharing for WebGL is already proposed, so the issue will probably fix itself in the future