compositor: Limit damage region to the screen

When an offscreen window is damaged, there is no need to repaint any
screen region.

Limit damage to the screen region only.

Signed-off-by: Olivier Fourdan <fourdan@xfce.org>
2 jobs for master in 2 minutes and 55 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #6688
build

00:01:15

 
  Distcheck
passed #6689
distcheck

00:01:40