function step() {
            if (self._running) {

                requestAnimationFrame(step);

                !self._paused && self._update();
            }
        }
Creat animator for a target, whose props can be animated.

@param  {Object} target
@param  {Object} options
@param  {boolean} [options.loop=false] Whether loop animation.
@param  {Function} [options.getter=null] Get value from target.
@param  {Function} [options.setter=null] Set value to target.
@return {module:zrender/animation/Animation~Animator}