Touchend is unreliable in multitouch #37

Trying mobile multitouch support, I've notice this unpredictable behaviour. Sometimes touchend is called, sometimes is not, and sometimes is called in a wrong x,y. Test code below and try it on mobile.

The code in the docs (handling multitouch in render()) runs perfectly, but there's no easy way to know if a touch starts or ends.

Tested on iPad mini (ios 7.1.2) and Nexus7 (android 5.0.2). On the nexus, where the frame rate is lower, the effect is worse.

var app = playground({
 ready: function(){
 this.layer.clear('#000').strokeStyle('#f00');
 },
 touchstart: function(ev) {
 this.layer.fillStyle("#0f0").fillCircle(ev.x, ev.y, 5);
 },
 touchmove: function(ev) {
 this.layer.fillStyle("#fff").fillCircle(ev.x, ev.y, 2);
 },
 touchend: function(ev) {
 this.layer.strokeCircle(ev.x, ev.y, 20); // <------ !!!
 }
});