// make sure requestAnimationFrame and cancelAnimationFrame are defined
// polyfill for browsers without native support
// by Opera engineer Erik Möller
var lastTime = 0;
var vendors = ['webkit', 'moz', 'ms', 'o'];
for(var x = 0; x < vendors.length && !window.requestAnimationFrame; ++x) {
    window.requestAnimationFrame = window[vendors[x]+'RequestAnimationFrame'];
    window.cancelAnimationFrame =
      window[vendors[x]+'CancelAnimationFrame'] || window[vendors[x]+'CancelRequestAnimationFrame'];
}
if (!window.requestAnimationFrame) {
    window.requestAnimationFrame = function(callback, element) {
        var currTime = new Date().getTime();
        var timeToCall = Math.max(0, 16 - (currTime - lastTime));
        var id = window.setTimeout(function() { callback(currTime + timeToCall); },
          timeToCall);
        lastTime = currTime + timeToCall;
        return id;
    }
}
if (!window.cancelAnimationFrame) {
    window.cancelAnimationFrame = function(id) {
        clearTimeout(id);
    }
}