Illustration of difference between:
touches(outer circle, white)
targetTouches(middle circle, green)
changedTouches(inner circle, red)
All events only bound to
button element, but once touched, touchpoints that didn't originate on the button (from the
touches list) will also appear.
Note that when a touch leaves the screen, its last known position is still retained in
changedTouches (leaving behind a red circle).