Set dpr of QImage in QOpenGLWidget grabFramebuffer

The QImage returned from grabFramebuffer should have the dpr of the
source.

Change-Id: Iafeabc1ab4e032fc28b73307104917ba3e898ad5
Reviewed-by: Laszlo Agocs <laszlo.agocs@theqtcompany.com>

src/widgets/kernel/qopenglwidget.cpp

@@ -758,6 +758,7 @@ QImage QOpenGLWidgetPrivate::grabFramebuffer()
     resolveSamples();
     q->makeCurrent();
     QImage res = qt_gl_read_framebuffer(q->size() * q->devicePixelRatio(), false, false);
+    res.setDevicePixelRatio(q->devicePixelRatio());
 
     return res;
 }