Optimize Item.transformOrigin implementation

As the default transformOrigin in QML is "Center", every time we change item geometry we recalculate the transformOriginPoint (which is the only concept understood by GraphicsView) and set it on the QGraphicsItem. This is especially costly the first time as it forces the creation of the transformData private member. Presumably this also has a cost at paint time. Most of the time the transformOriginPoint has no effect (if the item has no transforms), so this is an unnecessary cost.

On the n900, this amounts to 0.6ms over 11 items.