part of making BitmapWriteAccess an internal detail of vcl/

Change-Id: I8b3aa2fdd3c26db0e48b228640cd31b0bd31543c
Reviewed-on: https://gerrit.libreoffice.org/49937Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>