Position transform should include scroll offset

When using a morph as scrollable container, e.g. by using ```setClipMode('scroll')```, the transform does not account for the scroll offset. This causes the halos to be shown at the wrong position (see image below) and other glitches when using mouse events.

![scrolling](https://f.cloud.github.com/assets/479238/150368/1d7d5350-754f-11e2-900f-36d81a4d288d.png)