Extend QPixmap::grabWidget to grab OpenGL widget contents on Windows XP

@@ -102,8 +102,8 @@ void MainWindow::renderIntoPixmap()

 void MainWindow::grabFrameBuffer()
 {
-    QImage image = glWidget->grabFrameBuffer();
-    setPixmap(QPixmap::fromImage(image));
+    QPixmap pixmap = QPixmap::grabWidget(this);
+    setPixmap(pixmap);
 }

 void MainWindow::clearPixmap()