* clutter/clutter-backend.c:
* clutter/glx/clutter-backend-glx.c:
* clutter/glx/clutter-stage-glx.c:
Allow NULL to be passed to _clutter_backend_ensure_context
which essentially clears GL context. This is hooked into stage
unrealisation. Isn't yet quite bulletproof.
Fixes issues with gtk-embed crasher (thanks to Neil).