part of making GetAlpha/GetMask an internal detail of vcl

Change-Id: Ie1740df3d6435d9f0f2de3297e7f224d2c6b3cc5
Reviewed-on: https://gerrit.libreoffice.org/51436Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>