Qt4: only reset main interface window flags if needed

Unfortunately, according to the Qt4 documentation, changing the window
flags hides the window (until show() is called). So I guess there is no
way to toggle the on-top mode without the ugly blinking effect. Then,
try to minimize its occurences.

modules/gui/qt4/main_interface.cpp

@@ -1039,11 +1039,17 @@ void MainInterface::customEvent( QEvent *event )
     if ( event->type() == (int)SetVideoOnTopEvent_Type )
     {
         SetVideoOnTopQtEvent* p_event = (SetVideoOnTopQtEvent*)event;
+        Qt::WindowFlags oldflags = windowFlags(), newflags;
+
         if( p_event->OnTop() )
-            setWindowFlags( windowFlags() | Qt::WindowStaysOnTopHint );
+            newflags = oldflags | Qt::WindowStaysOnTopHint;
         else
-            setWindowFlags( windowFlags() & ~Qt::WindowStaysOnTopHint );
-        show(); /* necessary to apply window flags */
+            newflags = oldflags & ~Qt::WindowStaysOnTopHint;
+        if( newflags != oldflags )
+        {
+            setWindowFlags( newflags );
+            show(); /* necessary to apply window flags */
+        }
     }
 }