backend-glx: only check for GLX_SGI_video_sync for direct contexts
backend-glx: only check for GLX_SGI_video_sync for direct contexts

The GLX_SGI_video_sync spec explicitly says that it's only supported for
direct contexts so we don't setup up the function pointers if
glXIsDirect () returns GL_FALSE.

rkudiyarov/clutter_osx,djdeath/clutter-android,GNOME/clutter,GNOME/clutter,dlespiau/clutter,ebassi/clutter,jigpu/clutter,djdeath/clutter-android,dlespiau/clutter,ebassi/clutter,jigpu/clutter,kerrickstaley/clutter-vala,Distrotech/clutter,collects/clutter,ebassi/clutter,kerrickstaley/clutter-vala,nobled/clutter,Distrotech/clutter,jigpu/clutter,Distrotech/clutter,collects/clutter,djdeath/clutter-android,djdeath/clutter,dlespiau/clutter,Distrotech/clutter,GNOME/clutter,ebassi/clutter,jigpu/clutter,djdeath/clutter-multithreaded,collects/clutter,djdeath/clutter-multithreaded,heysion/clutter-clone,nobled/clutter,djdeath/clutter-multithreaded,rkudiyarov/clutter_osx,djdeath/clutter,ebassi/clutter,rkudiyarov/clutter_osx,djdeath/clutter,GNOME/clutter,spatulasnout/clutter,kerrickstaley/clutter-vala,GNOME/clutter,djdeath/clutter,kerrickstaley/clutter-vala,spatulasnout/clutter,djdeath/clutter-android,heysion/clutter-clone,heysion/clutter-clone,collects/clutter,Distrotech/clutter,jigpu/clutter,collects/clutter,djdeath/clutter-multithreaded,djdeath/clutter-android,nobled/clutter,rkudiyarov/clutter_osx,djdeath/clutter,spatulasnout/clutter,jigpu/clutter,djdeath/clutter,spatulasnout/clutter,Distrotech/clutter,heysion/clutter-clone,heysion/clutter-clone,spatulasnout/clutter