glx: Send GLX_VISUAL_SELECT_GROUP_SGIX attribute for visuals

We already send this for fbconfigs. Mesa happens to implement
glXChooseVisual relative to the fbconfig data, but that might not be
true of NVIDIA's libGL.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Adam Jackson <ajax@redhat.com>
(cherry picked from commit 43efaa6e)