part of making GetMask/GetAlpha an internal detail of vcl

Change-Id: I45c2e9fdae08d7f444a64e8e04a6f65bb525cbd1
Reviewed-on: https://gerrit.libreoffice.org/51417Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>