glx/dri: Filter out fbconfigs that don't have a supported pixmap format

Adam Jackson requested to merge ajax/xserver:dri-depth-30-fbconfig-fix into master

For depth 30 in particular it's not uncommon for the DDX to not have a configured pixmap format. Since the client expects to back both GLXPixmaps and GLXPbuffers with X Pixmaps, trying to use an x2rgb10 fbconfig would fail along various paths to CreatePixmap. Filter these fbconfigs out so the client can't ask for something that we know won't work.

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