glx: Fix glxUseXFont for glxWindow and glxPixmaps

glx: Fix glxUseXFont for glxWindow and glxPixmaps

The current implementation of glxUseXFont requires creating
a temporary pixmap and graphics context, which requires a real
old-school X11 Window, not a glxDrawable. This patch changes
things so that glxUseXFont will also accept a glxWindow or
glxPixmap, and lookup the underlying X11 Drawable. Without
this patch glxUseXFont generates a giant stream of Xerrors
about bad drawables and bad graphics contexts.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=54372

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Brian Paul <brianp@vmware.com>