There is a flashing effect we see on Mac when switching between
drawables (pbuffer and a window) on the same context.
This change make the FrameBuffer object in the renderer to use
two different OpenGL contexts, one which is used while the pbuffer
drawable is bound and another which is used when the subwindow is
bounded during swap. The two contexts share textures so both contexts
have access to all allocated color buffers.
While this fixes the flashing effect on Mac, it should be more effective on
other platforms as well.
Change-Id: I63b49f0cdae8e2785765503750bbf5fbd4d963a1
4629d2412b