fix full frame updates for VNC clients

If the client asks for !incremental frame updates, it has lost its content
so dirty doesn't matter - it has to see the full frame, so setting force_update

Signed-off-by: Stephan Kulow <coolo@suse.de>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Peter Lieven <pl@kamp.de>
diff --git a/ui/vnc.c b/ui/vnc.c
index 548588a..06d6ca0 100644
--- a/ui/vnc.c
+++ b/ui/vnc.c
@@ -1878,6 +1878,7 @@
         return;
     }
 
+    vs->force_update = 1;
     vnc_set_area_dirty(vs->dirty, width, height, x, y, w, h);
 }