part of making Bitmap an implementation detail of vcl/

Change-Id: Ic4b8d114a8091041374a083b3b7ca2fa68757ab1
Reviewed-on: https://gerrit.libreoffice.org/49719Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>