Add CC3GLBackgounder.

Add CC3GLBackgounder cluster. CC3OpenGL sharedGL singleton tracks separate instances
for rendering and background threads. CC3GLViewSurfaceManager tracks view. Manage
multiple GL contexts. Refactor CC3OSExtensions.h and related includes.
Repair confict between background and rendering contexts.

Remove VAO reference during VBO binding and move to 2D transition. Remove GL error
check during VBO binding. Repair strong references in CC3ModelSampleFactory.
In CC3DemoMashUp, move most of the scene creation to an asychronous background
thread, loading content while the scene is running.