Create abstract base class for OpenGLRenderer

This will eventually serve as a base class to allow
DisplayListRenderer to split off from OpenGLRenderer, and could
eventually support other rendering approaches, such as an
SkCanvas/SkPicture.

This will also be the main source of (implementation-independent)
documentation of the canvas/renderer methods.

Change-Id: I52047f338f5cf86a3b0b3002af7154bff5c3c227