grab() for QGraphicsView backed by QOpenGLWidget viewport

Attempting to grab() a QGraphicsView with QGLWidget viewport works. When migrating to QOpenGLWidget, it doesn't.

Even with fixes to fully offscreen QOpenGLWidget handling, this case is still broken due to the way QWidgetPrivate::drawWidget works and suppresses paint events for QOpenGLWidget and QQuickWidget. While this is desirable in most cases, not sending paints results in not rendering the graphics scene at all.