clutter/stage-cogl: Ensure queued_redraw_clip is not empty
When using CLUTTER_PAINT=damage-region
highlighting was conspicuously
absent during fullscreen animations like entering or leaving the
overview. That was because queued_redraw_clip
was empty, because it
had been initialized from redraw_clip == NULL
(full stage redraw).
Now we paint the damage region as the full view (which it is) instead of nothing at all.