Update QOpenGLWidget paintDevice devicePixelRatio when recreating fbo

The QOpenGLWidget's paintDevice needs to update devicePixelRatio when
changing screens.

Change-Id: Iec6e7157f22ff99e04f81347cbbf70f3fc3ff424
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>

src/widgets/kernel/qopenglwidget.cpp

@@ -558,6 +558,7 @@ void QOpenGLWidgetPrivate::recreateFbo()
     fbo->bind();
 
     paintDevice->setSize(deviceSize);
+    paintDevice->setDevicePixelRatio(q->devicePixelRatio());
 
     emit q->resized();
 }