part of a larger project to hide BitmapWriteAccess inside Bitmap

Change-Id: Ia46c12b3297107892a6236633c11ca9ada6edbd4
Reviewed-on: https://gerrit.libreoffice.org/49106Reviewed-by: Michael Meeks <michael.meeks@collabora.com>
Tested-by: Noel Grandin <noel.grandin@collabora.co.uk>