Composite windows from a QOpenGLFramebufferObject

This change introduces a mechanism for internal windows to be rendered
to a QOpenGLFramebufferObject to be composited using the texture bound
to the FBO. This is useful for in-process rendering (e.g. QtQuick) and
at the same time bypassing the windowing system.

The OpenGL context of the QOpenGLFramebufferObject needs to be sharing
with the compositing OpenGL context.