part of the process of making Bitmap an internal detail of vcl/

and remove last outside-of-vcl use of BitmapWriteAccess from
header file.

Change-Id: I59a4353e11f881d24a7be50cf0f91f4f0dcffcaa
Reviewed-on: https://gerrit.libreoffice.org/49850Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>