bnc#719994: We need the proper alpha mask

Change-Id: Ic16dceaca431929238fcef6be31ee95e068b91e4
(cherry picked from commit e06014e0db64ec36ec41dee542d2453c945fddd3)
Reviewed-on: https://gerrit.libreoffice.org/11128
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>