[Renderer] Investigate issue with rendering trasparent/semi-transparent objects with different z-order #18

Investigate issue with rendering trasparent/semi-transparent objects with different z-order.
Possible solutions:
For 2D Renderer (with disabled depth testing):

• (PRIMARY OPTION) Layers system
• Just render in the correct order (sorting) according to Sprite Z position before rendering.

For 3D Renderer:

• Make next rendering order:
  • Draw all opaque objects first.
  • Sort all the transparent objects.
  • Draw all the transparent objects in sorted order
• Order independent transparency technique. Good explanation here