Revert "dont use GetMask in GeoTexSvxBitmapEx"

This reverts commit 63e65d17.

Until we come up with something better

Change-Id: I246468abdd5e3ee917143e251c2e95430d84f77b
Reviewed-on: https://gerrit.libreoffice.org/55385Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>