sdl2: redraw correctly when scanout_mode enabled.

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

ui/sdl2-gl.c

@@ -124,6 +124,11 @@ void sdl2_gl_redraw(struct sdl2_console *scon)
 {
     assert(scon->opengl);
 
+    if (scon->scanout_mode) {
+        /* sdl2_gl_scanout_flush actually only care about
+         * the first argument. */
+        return sdl2_gl_scanout_flush(&scon->dcl, 0, 0, 0, 0);
+    }
     if (scon->surface) {
         sdl2_gl_render_surface(scon);
     }