code: function getPointByClient(clientX, clientY) {
      var el = this.get('el');
      var pixelRatio = this.get('pixelRatio') || 1;
      var bbox = el.getBoundingClientRect();
      return {
        x: (clientX - bbox.left) * pixelRatio,
        y: (clientY - bbox.top) * pixelRatio
      };
    }
docs: 将窗口坐标转变成 canvas 坐标
  @param  {Number} clientX 窗口x坐标
  @param  {Number} clientY 窗口y坐标
  @return {Object} canvas坐标