XShmGetImage: fix censoring

It looks like offsets calculated during image censoring are wrong.
This results in black (empty) images returns.

This fix is very similar to 6c6f09aa
that was applied to XGetImage

Visually this fixes chromium/firefox window sharing in multiscreen
configurations - without this patch most of the windows on 'secodnary'
screens are black.

This also should fix https://bugs.freedesktop.org/show_bug.cgi?id=101730

.

Signed-off-by: Nikolay Martynov <mar.kolya@gmail.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
(cherry picked from commit 885636b7)