ui/gtk: Add keep-aspect-ratio option

ui/gtk: Add keep-aspect-ratio option

When aspect ratio of host window and that of guest display are not
aligned, we can either zoom the guest content to fill the whole host
window or add padding to respect aspect ratio of the guest. Add an
option keep-aspect-ratio to allow users to select their preferred
behavior in this case.

Suggested-by: BALATON Zoltan <balaton@eik.bme.hu>
Suggested-by: Kim, Dongwon <dongwon.kim@intel.com>
Signed-off-by: Weifeng Liu <weifeng.liu.z@gmail.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Tested-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20250601045245.36778-2-weifeng.liu.z@gmail.com>