QGraphicsView - fix rubberband to stay right on scroll

QGraphicsView - fix rubberband to stay right on scroll

We should consider the scene-position when we are expanding
moving a rubberband. If the user does some auto-scroll
(Qt should support that itself, but that is another matter)
then the rubberband should not keep the (old) local position
to calculate the rubberband extension, but instead use the
scene-position that was actually clicked.

Change-Id: I04a2df6a1edae8b3587e1ac2104c7fe4ccfb7762
Reviewed-by: Andy Shaw <andy.shaw@digia.com>
Reviewed-by: Andreas Aardal Hanssen <andreas@hanssen.name>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>