demos: Fix vsync checking in glxgears

demos: Fix vsync checking in glxgears

Of course glXGetVideoSyncSGI doesn't return the swap interval.  The feature
only exists in the Mesa extension... which is the whole reason I created the
Mesa extension!  Note that the Mesa extension allows drivers to default to a
swap interval of 0.  If the Mesa extension exists, use its value.  Only
consider the SGI extension when the Mesa extension is not available.

Fixes bug #22604.