the master page is not the sole owner. Which happens when copying
and pasting slides which bring along a duplicate master page to
an already existing one, and the attempt to remove the duplicate
strips the fill properties from the shared stylesheet in use by
the other

regression from...

commit b876bbe2
Author: David Tardon <dtardon@redhat.com>
Date:   Tue Apr 26 09:17:11 2016 +0200

    rbhz#1326602 avoid exp. bg bitmaps from deleted slides

Change-Id: I91fb8f622a0e35741ecc37cef14fc93199bb730b