part of making GetAlpha/GetMask an internal detail of vcl

Change-Id: I874a68f340cd3074cfbeb6303f52adeeb13e56a5
Reviewed-on: https://gerrit.libreoffice.org/51435Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>