Revert "Don't rely on mapFromGlobal in QGraphicsScenePrivate::itemsAtPosition."

Revert "Don't rely on mapFromGlobal in QGraphicsScenePrivate::itemsAtPosition."

This reverts commit 7c0d15a22266a425c9e9ac0120d6774e120fe01e.
The commit caused a regression whereby tooltips may be shown even if the
mouse is not over the item if it has the Qt::ItemIgnoresTransformations
flag and the QGraphicsView had been scaled.

Task-number: QTBUG-17517
Task-number: QTBUG-22663
Change-Id: Ib7fd788d9712c5e659fe07182f9505a4eb135ab2
Reviewed-by: Andy Shaw <andy.shaw@digia.com>
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>
(cherry picked from commit 15c14584199dc43e4a309fc331f3144009008128)