drisw: Cache the depth of the X drawable

drisw: Cache the depth of the X drawable

This is not always ->rgbBits, because there are cases where that could
be 32 but we're (legally) bound to a depth-24 pixmap. The important
thing to have match here is the actual server-side notion of depth.  You
can look this up (at modest expense) from the xlib visual info if the
fbconfig has a visual. But it might not, so if not, fetch it (at
slightly greater expense) from XGetGeometry. Do this at GLX drawable
creation so you don't have to do it on the SwapBuffers path.

Apparently this fixes glx/glx-swap-singlebuffer, which is unintentional
but quite pleasant.

Fixes: mesa/mesa#2291
Fixes: 90d58286 ("drisw: Fix and simplify drawable setup")
Reviewed-by: Eric Anholt <eric@anholt.net>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3305>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3305>