glXChooseVisual: Only consider fbconfig if we can get the corresponding visual.
glXChooseVisual: Only consider fbconfig if we can get the corresponding visual.

This can fail, e.g. when XLIB_SKIP_ARGB_VISUALS=1 is set.

See http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=524794 and
http://bugs.freedesktop.org/show_bug.cgi?id=21600 .

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