code: function releaseFrame(frame) {
              var index = pendingFrames.indexOf(frame);
              if (~index) {
                  pendingFrames.splice(index, 1);
              }
              if (pendingFrames.length === 0) {
                  cancelAnimationFrame(frameRequestId);
              }
          }
docs: @method kity.releaseFrame
  @catalog animate
  @grammar kity.releaseFrame(frame)
  @description 释放一个已经请求过的帧，如果该帧在等待集合里，将移除，下个动画帧不会执行释放的帧
  
  @param {frame} frame 使用 kity.requestFrame() 返回的帧
  
  @example
  
  ```js
  var frame = kity.requestFrame(function() {....});
  kity.releaseFrame(frame);
  ```