fix for scrolled documents

Was not finding the drop target on scrolled documents due to incorrect viewport coordinates. Problem can be seen scrolling demo slightly off top of screen and observing what happens dropping in bottom of drop area or just outside top.

Subtracting the scroll offsets returns the correct viewport coordinates. Tested on several ipads and an android device.

This is probably only good for ios 5+ as the article at http://www.icab.de/blog/2011/10/17/elementfrompoint-under-ios-5/ says the behaviour of elementFromPoint changed at that point. I don't have anything I haven't updated so I couldn't easily test it.