ui/gtk-gl-area: Render guest content with padding in fixed-scale mode

ui/gtk-gl-area: Render guest content with padding in fixed-scale mode

In fixed-scale mode (zoom-to-fit=false), we expect that scale should not
change, meaning that if window size is larger than guest surface,
padding is supposed to be added to preserve the scale. However, in
OpenGL mode (gl=on), guest surface is always painted to the whole canvas
without any padding. This change tries to fix this bug by adding
appropriate padding when drawing surfaces.

Signed-off-by: Weifeng Liu <weifeng.liu.z@gmail.com>
Message-ID: <20250511073337.876650-9-weifeng.liu.z@gmail.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Marc-André Lureau <marcandre.lureau@redhat.com>