Create layering framework for visuals

[cdsupina]

The layering of visuals in the game is all over the place. There are some "z level" constants in constants/mod.rs. Some visuals are hard-coded with a z level. We should have solidly defined z levels (layers) for every visual in the game with a sensible way of accessing the layering data (maybe a resource?).

We also need to decide in what order should visuals appear. (should enemies appear above bosses?, should blasts appear above consumables?, should the player appear below blasts? effects?)