rasterwindow: End painting on device before ending on backingstore

Change-Id: Ib42de1d7ad0d14e2e2d2ff75606e6cfd7e584c2a
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>

examples/gui/rasterwindow/rasterwindow.cpp

@@ -111,6 +111,7 @@ void RasterWindow::renderNow()
 
     painter.fillRect(0, 0, width(), height(), Qt::white);
     render(&painter);
+    painter.end();
 
     m_backingStore->endPaint();
     m_backingStore->flush(rect);