Fix tst_qgraphicseffect on Wayland

If we only wait until the window is shown before testing the
event delivery, we may be getting a WindowActivation event
and subsequent paint event after we start testing. This can
cause a shift in the paint events, so that we end up with an
unexpected paint event at the end.

We should wait until the window is active before we start
checking the response to updates, to be sure nothing is pending
on the queue still.

Note that you'd expect QTest::qWaitForWindowActive() to do this,
but this actually falls back to qWaitForWindowExposed() when
the platform does not have WindowActivation capability. While
there is no real link between WindowActivation capability and
waiting for a window to be active, changing the behavior of that
function would be too scary, so we just implement an explicit
wait in the functions that depend on this.

Task-number: QTBUG-91418
Change-Id: Iee40dcfa1377f543ea05042cc5a972270b346708
Reviewed-by: Paul Olav Tvete <paul.tvete@qt.io>

tests/auto/widgets/effects/qgraphicseffect/tst_qgraphicseffect.cpp

@@ -694,6 +694,7 @@ void tst_QGraphicsEffect::prepareGeometryChangeInvalidateCache()
     QGraphicsView view(&scene);
     view.show();
     QVERIFY(QTest::qWaitForWindowExposed(&view));
+    QTRY_VERIFY(view.windowHandle()->isActive());
     QTRY_VERIFY(item->nbPaint >= 1);
 
     item->nbPaint = 0;