QWidget: Don't offset the clip region for windows using effects

When a top-level widget has a graphic effect applied,
QWidgetPrivate::clipToEffectiveMask() must not use the parent widget for
the mask calculation as this prevents the loop below from bailing out in
the first loop (w->isWindow()) and might mess up the clip region when
there is a widget with a mask in the parent chain.

Pick-to: 6.9 6.8
Fixes: QTBUG-131893
Change-Id: I5d5777e730bee309db21ad5eb65387d19321ee6c
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/widgets/kernel/qwidget.cpp

@@ -2113,7 +2113,7 @@ void QWidgetPrivate::clipToEffectiveMask(QRegion &region) const
     QPoint offset;
 
 #if QT_CONFIG(graphicseffect)
-    if (graphicsEffect) {
+    if (graphicsEffect && !w->isWindow()) {
         w = q->parentWidget();
         offset -= data.crect.topLeft();
     }