ui/gtk: Update scales in fixed-scale mode when rendering GL area

ui/gtk: Update scales in fixed-scale mode when rendering GL area

When gl=on, scale_x and scale_y were set to 1 on startup that didn't
reflect the real situation of the scan-out in free scale mode, resulting
in incorrect cursor coordinates to be sent when moving the mouse
pointer. Simply updating the scales before rendering the image fixes
this issue.

Signed-off-by: Weifeng Liu <weifeng.liu.z@gmail.com>
Message-ID: <20250511073337.876650-5-weifeng.liu.z@gmail.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Marc-André Lureau <marcandre.lureau@redhat.com>
(cherry picked from commit 8fb072472c38cb1778c5b0bebf535a8b13533857)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>