QRegion::subtract is deprecated

This code was suspicious. It looks like it was broken, caught by the
gotcha that the old API was.

Change-Id: Ief9543de3b9fdf18f6b620c9462a75594a4a3903
Reviewed-by: Dmytro Poplavskiy <dmytro.poplavskiy@nokia.com>

examples/videowidget/videowidget.cpp

@@ -89,7 +89,7 @@ void VideoWidget::paintEvent(QPaintEvent *event)
 
         if (!videoRect.contains(event->rect())) {
             QRegion region = event->region();
-            region.subtract(videoRect);
+            region = region.subtracted(videoRect);
 
             QBrush brush = palette().background();