Render everything in layer order instead of using sprites

I believe this should fix almost all layer rendering errors. The only thing that could be improved is clearing the layer list on a scene change. Although, it probably won't cause much of a performance hit since every layer tracks if it is dirty or not.

The one exception is Text which I will probably change back to using a BitmapData object instead of using native text rendering. There are too many limitations to the current method; layering, rotation, scaling, etc...

My goal is to release this as 2.5.0 since it changes the underlying structure of HaxePunk. Let me know if you find issues with it.