The XVisualInfo for GL is created when a stage is being realized.
When embedding Clutter inside another toolkit we might not want to
realize a stage to extract the XVisualInfo, then set the stage
window using a foreign X Window -- which will cause a re-realization.

Instead, we should abstract as much as possible into the X11 backend.

Unfortunately, the XVisualInfo for GL is requested using GLX API; for
this reason we have to create a ClutterBackendX11 method that we
override inside the ClutterBackendGLX implementation.

This also allows us to move a little bit of complexity from out of
the stage realization, which is currently a very delicate and hard
to debug section.