Add missing flush for multisampled QOpenGLWidget

QOpenGLWidget exhibits the same issue as QQuickWidget in the linked bug.

Task-number: QTBUG-39917
Change-Id: Ib231fb88f73c6ef68f12cc3fecf462679e8184a7
Reviewed-by: Jørgen Lind <jorgen.lind@theqtcompany.com>

src/widgets/kernel/qopenglwidget.cpp

@@ -768,6 +768,7 @@ void QOpenGLWidgetPrivate::resolveSamples()
         q->makeCurrent();
         QRect rect(QPoint(0, 0), fbo->size());
         QOpenGLFramebufferObject::blitFramebuffer(resolvedFbo, rect, fbo, rect);
+        QOpenGLContext::currentContext()->functions()->glFlush();
     }
 }