canvas pans when tap on touch screen
reason:
onTouchStart calls onMouseDown which registers 'mousemove' event handler.
when I tap on touch screen, even though browser did not dispatch 'touchmove' event, it dispatched 'mousemove' event.
therefore, the 'mousemove' handler pans canvas.
fix:
stop onTouchStart handler registering 'mousemove' handler as onMouseMove will be properly triggered by onTouchMove
0bb9c5be97