Fix: make event.target the actual target where pan started

[zoexi]

Hi, this is a Fix for Issue #815. I have tested the changes on Chrome (49.0.2623.87), Firefox (44.0.2), and Safari (9.0.3) using the demo kindly provided by @LeJared. Changes are local to only src/recognizers/pan.js and tests/manual/panstart.html.

Any feedback is appreciated! Please let me know if I should change or clarify anything. Thanks so much for your time. :)

Before, event.target is set to the element where the mouse pointer is at, after it has moved at least PanRecognizer.options.threshold pixels away from the mousedown position. This would normally not be a problem if the target element's size is greater than threshold pixels, as the mouse pointer would still be within the target element at the end of the pan gesture, thus event.target would still be set to the expected element.

However, if the element we originally panned over is smaller than threshold pixels, by the time the pan gesture has been recognized, the mouse pointer would be over a different element, not the original element of panstart (not what we want).

Now, the event.target of the pan gesture is set to where the user originally clicked (on panstart), as expected. This is done by saving the original target element, and updating the input before emitting it to the listeners.