Cocoa: Fix QCocoaBackingStore::flush on 10.8

Cocoa: Fix QCocoaBackingStore::flush on 10.8

The call to displayRect works on 10.7 but does not
update the pixels on screen on 10.8. Many Qt windows
do not update properly.

This is a workaround found by trail and error
without fully understanding the underlying issue.
I would like to have this in place until we can
determine the cause of the bug.

Change-Id: I1ecee745f42a93ded4f651cc8ad51963f250e78d
Reviewed-by: Christoph Schleifenbaum <christoph.schleifenbaum@kdab.com>
Reviewed-by: Bradley T. Hughes <bradley.hughes@nokia.com>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@nokia.com>