Draft: composite: Mark the COW unhittable
The composite overlay window (COW) can be queried from any X11 client, not just the X11 compositing manager.
If a client tries to get the composite overlay window, the Xserver will map the window and it will get all pointer events (the window being mapped and on top of the stack).
To avoid that issue, mark the composite overlay window as unhittable.
Closes: #1314 (closed)
Signed-off-by: Olivier Fourdan ofourdan@redhat.com