fdo#79615 create clipping polygon correctly

Change-Id: I758b6bc8da33a20a0807da171120a8f84214dcfd
(cherry picked from commit 8fae55c7d3fead6ce18556d5ccaa0958b7cfed91)
Reviewed-on: https://gerrit.libreoffice.org/9864
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>