When a drawable goes away, we don't destroy the GLX drawable in full,
since it may be current for a context. This means that when the drawable
is destroyed in full later, the backend doesn't get a chance to
destroy resources associated with the drawable (the DRI2Drawable).
With this patch, we destroy the GLX drawable in full when it goes away
and then track down all contexts that reference it and NULL their
pointers.
(cherry picked from commit 7b6400a1b8)
