If the client asks for !incremental frame updates, it has lost its content
so dirty doesn't matter - it has to see the full frame, so setting force_update
Signed-off-by: Stephan Kulow <coolo@suse.de> Signed-off-by: Gerd Hoffmann <kraxel@redhat.com> Reviewed-by: Peter Lieven <pl@kamp.de>