code: function _getMatrixByRoot(root) {
      var self = this;
      root = root || self;
      var parent = self;
      var parents = [];
  
      while (parent !== root) {
        parents.unshift(parent);
        parent = parent.get('parent');
      }
      parents.unshift(parent);
  
      var m = [1, 0, 0, 0, 1, 0, 0, 0, 1];
      Util.each(parents, function (child) {
        mat3.multiply(m, child.attr('matrix'), m);
      });
      return m;
    }
docs: 获取到达指定根节点的矩阵