part of making mask and alpha internal details of Bitmap/BitmapEx

Change-Id: I87ca24af18a29f5eb8a5761c5d95ae2806d97e77
Reviewed-on: https://gerrit.libreoffice.org/55078Reviewed-by: Michael Meeks <michael.meeks@collabora.com>
Tested-by: Jenkins <ci@libreoffice.org>