Wait for window expose on the viewport widget

The child viewport window covers the entire client
area of the parent window, which prevents the parent
window from being exposed.

This worked by accident until now, and was uncovered
by changes to the cocoa platform plugin implementation.

Change-Id: I0d62a1a33f125e5a9030c8def452a49886e54956
Task-number: QTBUG-50414
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

tests/auto/opengl/qgl/tst_qgl.cpp

@@ -868,7 +868,7 @@ void tst_QGL::graphicsViewClipping()
 
     scene.setSceneRect(view.viewport()->rect());
 
-    QVERIFY(QTest::qWaitForWindowExposed(&view));
+    QVERIFY(QTest::qWaitForWindowExposed(view.viewport()));
     #ifdef Q_OS_MAC
         // The black rectangle jumps from the center to the upper left for some reason.
         QTest::qWait(100);