compositor: Ignore opaque region for shaded windows

Typically, the opaque region applies to the client window.

When shaded, the client window is unmapped and the frame shrunk to its
minimum height.

When shaded, the opaque regio nshould simply be ignored as the client
window is no longer visible.

Signed-off-by: Olivier Fourdan <fourdan@xfce.org>
Closes: #452
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passed #6318
build

00:01:24

 
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passed #6319
distcheck

00:01:39