cogl-texture-pixmap-x11: Use NULL instead of None for invalid GLXFBConfig
cogl-texture-pixmap-x11: Use NULL instead of None for invalid GLXFBConfig

GLXFBConfigs are opaque pointers not XIDs so it doesn't make sense to
return None from get_fbconfig_for_depth. Instead it now returns NULL.

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