QGLWidget and re-parenting

On OS X we have QWidgetPrivate::glWidgets list to track all child gl-widgets
and this propagates though all the widget's tree, starting from QGLWidget and up to the
top-level window ('isWindow'). It can happen, that some of widgets in a chain is becoming
a window (but still has a parent) - this results in QGLWidget's destructor failing to remove
'this' from one of ancestors (since it stops on the first 'isWindow' ancestor).
Modify QWidgetPrivate::setParent_sys that it does NOT add glWidgets into the parent's
glWidgets, if this widget isWindow itself.

Change-Id: I4bf4a927a4e43537355fdcdef465638c91e9f956
Task-number: QTBUG-20652
Reviewed-by: Gabriel de Dietrich <[email protected]>

Author: Timur Pocheptsov

src/gui/kernel/qwidget_mac.mm

@@ -3092,7 +3092,7 @@ void qt_mac_updateParentUnderAlienWidget(QWidget *alienWidget)
 
     // Maintain the glWidgets list on parent change: add "our" gl widgets
     // to the list on the new parent and grandparents.
-    if (glWidgets.isEmpty() == false) {
+    if (glWidgets.isEmpty() == false && !q->isWindow()) {
         QWidget *current = q->parentWidget();
         while (current) {
             current->d_func()->glWidgets += glWidgets;