tst_QWidget::gdiPainting fixed

We cannot use QWidget::grab for widgets that have WA_PaintOnScreen set.
QScreen::grabWindow copies the real screen contents for us.

Change-Id: If1f6233ec48bcb2b941ea683c56ce71a39642e67
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>

tests/auto/widgets/kernel/qwidget/tst_qwidget.cpp

@@ -7844,7 +7844,8 @@ public:
 
 private slots:
     void slotTimer() {
-        const QImage im = grab(QRect(QPoint(0, 0), size())).toImage();
+        QScreen *screen = windowHandle()->screen();
+        const QImage im = screen->grabWindow(internalWinId(), 0, 0, -1, -1).toImage();
         color = im.pixel(1, 1);
         accept();
     }