Constraints: Dragged elements are stuck when constrained widget shrinks

[caitp]

The bug is visible on on http://caitp.github.io/angular-drop/ when the dragged element is moved to the far right side of the constraint parent, and then resizing the browser so that the dragged element is outside of the constraints.

I think it may be necessary to handle viewport resize events, which could be tricky. A timeout mechanism could also work. I'll go with the event handler first, though.