ui/gtk: set the area of the scanout texture correctly

ui/gtk: set the area of the scanout texture correctly

x and y offsets and width and height of the scanout texture
is not correctly configured in case guest scanout frame is
dmabuf.

Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Marc-André Lureau <marcandre.lureau@redhat.com>
Cc: Vivek Kasireddy <vivek.kasireddy@intel.com>
Signed-off-by: Dongwon Kim <dongwon.kim@intel.com>
Message-ID: <20230621213150.29573-1-dongwon.kim@intel.com>
(cherry picked from commit 37802a24eb4e535d96d6fe0273505d2b5c6528c2)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>