When scanout_mode enabled, surface is out of sync with actual screen.
In such case, we just call sdl2_gl_scanout_flush to do redraw. This
fixes bug reported in
https://lists.freedesktop.org/archives/virglrenderer-devel/2018-July/001330.html



Signed-off-by: Tao Wu <lepton@google.com>
Message-id: 20180726225900.180698-1-lepton@google.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>