compositor: Update composite overlay window before unredirecting

The current ordering updates the clip shape of the composite overlay
window after unredirecting the target window. This has the effect of
forcing X to clear the target window and sending an expose to the
application to repaint - causing an unsightly flash. If we update the
shape first, then unredirect, X restores the background of the root
window (sending no expose events as no one is interested) and the
background is typically NONE for the root window. Then the unredirect
paints the contents of the composite backing pixmap over top without
requiring a round trip and waiting for the client to repaint - thus no
flashing.

Fixes regression from

commit d6282716


Author: Jasper St. Pierre <jstpierre@mecheye.net>
Date:   Fri Dec 6 17:10:44 2013 -0500

    compositor: Simplify the unredirected window management code

Cc: Jasper St. Pierre <jstpierre@mecheye.net>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

https://bugzilla.gnome.org/show_bug.cgi?id=743858