vcl: remove ::Bitmap as friend of OutputDevice

Change-Id: Idea0fee1ca79ae1660e89ef7038b35cf27358082
Reviewed-on: https://gerrit.libreoffice.org/13013
Reviewed-by: Chris Sherlock <chris.sherlock79@gmail.com>
Tested-by: Chris Sherlock <chris.sherlock79@gmail.com>