Better multitouch.

This fixes a bug where all touches would ending on any touch ending.

The drag behavior registers a touchend listener for each started touch; however, a touchend event is dispatched to ALL listeners when any touch ends, not just for the corresponding starting touch. The drag behavior must therefore detect whenever the ending touch is the corresponding starting touch, and ignore other ending touches.

This fixes the drag behavior during multitouch, as discussed in #1786.