When using OpenGL contexts created externally, vtkOpenGLRenderWindow may
encounter scenarios where an FBO is being used, instead of the default
frame buffer to render into. When that's the case, we need to activate
that FBO before rendering.

Adding a mechanism to vtkOpenGLRenderWindow to:
1. discover and save the id for the default fb being used.
2. before rendering, ensure that the non-default fb is activated in
   vtkOpenGLCamera and vtkCameraPass.