ui: honour the actual guest display dimensions without rounding

ui: honour the actual guest display dimensions without rounding

A long time ago the VNC server code had some memory corruption
fixes done in:

  commit bea60dd7679364493a0d7f5b54316c767cf894ef
  Author: Peter Lieven <pl@kamp.de>
  Date:   Mon Jun 30 10:57:51 2014 +0200

    ui/vnc: fix potential memory corruption issues

One of the implications of the fix was that the VNC server would have a
thin black bad down the right hand side if the guest desktop width was
not a multiple of 16. In practice this was a non-issue since the VNC
server was always honouring a guest specified resolution and guests
essentially always pick from a small set of sane resolutions likely in
real world hardware.

We recently introduced support for the extended desktop resize extension
and as a result the VNC client has ability to specify an arbitrary
desktop size and the guest OS may well honour it exactly. As a result we
no longer have any guarantee that the width will be a multiple of 16,
and so when resizing the desktop we have a 93% chance of getting the
black bar on the right hand size.

The VNC server maintains three different desktop dimensions

 1. The guest surface
 2. The server surface
 3. The client desktop

The requirement for the width to be a multiple of 16 only applies to
item 2, the server surface, for the purpose of doing dirty bitmap
tracking.

Normally we will set the client desktop size to always match the server
surface size, but that's not a strict requirement. In order to cope with
clients that don't support the desktop size encoding, we already allow
for the client desktop to be a different size that the server surface.

Thus we can trivially eliminate the black bar, but setting the client
desktop size to be the un-rounded server surface size - the so called
"true width".

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20210311182957.486939-5-berrange@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>