skins2: fix a refresh artefact (not frequent corner case)

If, for instance, video is not embedded and visible="vlc.hasVout" is used
for a video control, the video control background was not refreshed.
Yet, partial redraw (overlapping windows) could occur, leading to a visual artefact.
(cherry picked from commit a8fcb7dd6c253e8534d8582b4ef2f9ba30fae691)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/gui/skins2/controls/ctrl_video.cpp

@@ -154,6 +154,7 @@ void CtrlVideo::onUpdate( Subject<VarBool> &rVariable, void *arg  )
     {
         msg_Dbg( getIntf(), "VideoCtrl : Visibility changed (visible=%d)",
                                   isVisible() );
+        notifyLayout();
     }
 
     // Active Layout changed