part of making Bitmap an implementation detail of vcl/

Change-Id: I847a2371cd630637b11aed0fad7263809143c706
Reviewed-on: https://gerrit.libreoffice.org/49705Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>