glx: Require screens match for share contexts for classic CreateContext

The GLX_ARB_create_context path (with which this should all get unified,
someday, sigh) already enforces this, but the classic path does not.
It's effectively assumed by the implementation anyway, so let's enforce
it rather than do crashy things.

Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
Signed-off-by: Adam Jackson <ajax@redhat.com>
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