Add a warning when using QOpenGLWidget as a native child

Just like it is done for QQuickWidget.

Task-number: QTBUG-41779
Change-Id: I1b27c2ed34ecb2520edf82843b675dbf6b0eab8e
Reviewed-by: Michael Bruning <michael.bruning@digia.com>
Reviewed-by: Paul Olav Tvete <paul.tvete@digia.com>

src/widgets/kernel/qopenglwidget.cpp

@@ -506,6 +506,12 @@ void QOpenGLWidgetPaintDevice::ensureActiveTarget()
 
 GLuint QOpenGLWidgetPrivate::textureId() const
 {
+    Q_Q(const QOpenGLWidget);
+    if (!q->isWindow() && q->internalWinId()) {
+        qWarning() << "QOpenGLWidget cannot be used as a native child widget."
+                   << "Consider setting Qt::AA_DontCreateNativeWidgetAncestors and Siblings.";
+        return 0;
+    }
     return resolvedFbo ? resolvedFbo->texture() : (fbo ? fbo->texture() : 0);
 }