Refactor OpenGLRenderer to simplify OpenGL drawing code.

This is the first step of the refactoring:
- drawColorRect
- drawTextureMesh

Remaining methods to refactor:
- setupTextureAlpha8
- drawPath
- setupShadow
- drawText
- drawLines
- composeLayerRegion

Change-Id: I0fced6a05b6b6eee10ee7cef0c3fbdaa6cefab20