ui/gtk: set scanout-mode right before scheduling draw

ui/gtk: set scanout-mode right before scheduling draw

Setting scanout mode is better to be done very last minute
right because the mode can be reset anytime after it is set in
dpy_gl_scanout_texture by any asynchronouse dpy_refresh call,
which eventually cancels drawing of the guest scanout texture.

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>
Acked-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-ID: <20230706183355.29361-1-dongwon.kim@intel.com>