Fix handling of touchmove events on Android

Dragging the canvas did not work on Android devices as the === check for
lastTouch and the current event was always evaluating to false.
Presumably Safari on iOS re-uses the same Touch object for touchmove
events with the same finger, whereas Chrome/Firefox on Android creates
new Touch objects for each event (so the === evaluates false). The code
now compares Touch.identifier to ensure the new touch event is from the
same finger as the initiating touchstart.