ui/vnc: fix vmware VGA incompatiblities

this fixes invalid rectangle updates observed after commit 12b316d4
with the vmware VGA driver. The issues occured because the server
and client surface update seems to be out of sync at some points
and the max width of the surface is not dividable by
VNC_DIRTY_BITS_PER_PIXEL (16).

Reported-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Signed-off-by: Peter Lieven <pl@kamp.de>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>