since we hold it like that in Bitmap anyway

Change-Id: I6264dfaaae6210cb008df5db8a421fc80c508f5b
Reviewed-on: https://gerrit.libreoffice.org/55458Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>