Feature: support sprite z-order (sorting)

[ivan-mogilko]

I think the reason why this plugin did not support this from the start is the fact that the engine itself does not use z coordinate at all, and all of its textures are rendered at z = 0 (engine sorts the sprites itself before passing into renderer).

If this plugin would use z coordinate, that would not work at all (due to perspective / clipping settings applied by the engine), or will break sorting between plugin's and engine's render stages.

One solution is to do same thing as engine is currently doing: that is sort the group of sprites ourselves (per render stage).