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.
GLX is enabled by default, but the current swrast behaviour causes X
to abort with fatal error if the swrast dri library dlopen fails.
Handle the case where the swrast library is not present, and do not
register the GLX extension unless at least one screen has a usable
GL provider.