gdk/stage: Warn if CoglContext is not available
gdk/stage: Warn if CoglContext is not available

Instead of crashing inside Cogl later on.

https://bugzilla.gnome.org/show_bug.cgi?id=745229

jigpu/clutter,jigpu/clutter,GNOME/clutter,jigpu/clutter,GNOME/clutter,jigpu/clutter,jigpu/clutter,Distrotech/clutter,GNOME/clutter,Distrotech/clutter,Distrotech/clutter,Distrotech/clutter,Distrotech/clutter,Distrotech/clutter,GNOME/clutter,GNOME/clutter,jigpu/clutter