Bug 1200 - Crash with invalid DISPLAY
* clutter/clutter-main.c: Use the ClutterBackend wrapper API
instead of directly checking the class structure.
* clutter/glx/clutter-backend-glx.c: Return the correct value
in case of failure.
* clutter/x11/clutter-backend-x11.c: Bail out if XOpenDisplay()
failed.