glx: Clean up sync-to-vblank mechanism discovery
glx: Clean up sync-to-vblank mechanism discovery

The code has gotten really complicated to follow.

As soon as we have a sync-to-vblank mechanism we should just bail out.

Also, __GL_SYNC_TO_VBLANK (which is used by nVidia) should be assumed
equivalent to a CLUTTER_VBLANK_GLX_SWAP.

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