QMacStyle: fix quadratic behavior

Repeatedly calling QVector::erase(it) (via QMutableVectorIterator::remove())
results in quadratic runtime.

Separate the removal of expired objects from the sending of the event to
the remaining objects, so we can use QVector::removeAll() for the former,
which does exactly what the old code tried to do, except in linear time.

Use range-for for the sending-loop. This could cause detaches, but since
we modify the container two lines before, and we don't copy it anymore,
anywhere, detaches are impossible.

Change-Id: I9aa6427e3646c8ad92b673fe42a86a0dfe79ee80
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>

src/widgets/styles/qmacstyle_mac.mm

@@ -118,14 +118,13 @@ QT_NAMESPACE_ALIAS_OBJC_CLASS(NotificationReceiver);
 - (void)scrollBarStyleDidChange:(NSNotification *)notification
 {
     Q_UNUSED(notification);
+
+    // purge destroyed scroll bars:
+    QMacStylePrivate::scrollBars.removeAll(QPointer<QObject>());
+
     QEvent event(QEvent::StyleChange);
-    QMutableVectorIterator<QPointer<QObject> > it(QMacStylePrivate::scrollBars);
-    while (it.hasNext()) {
-        if (!it.next())
-            it.remove();
-        else
-            QCoreApplication::sendEvent(it.value(), &event);
-    }
+    for (const auto &o : QMacStylePrivate::scrollBars)
+        QCoreApplication::sendEvent(o, &event);
 }
 @end