code: function(offset) {
          var index = this._indexArray,
              l = this.index_pointer;
          index[0 + l] = 0 + offset;
          index[1 + l] = 1 + offset;
          index[2 + l] = 2 + offset;
          index[3 + l] = 1 + offset;
          index[4 + l] = 2 + offset;
          index[5 + l] = 3 + offset;
          this.index_pointer += 6;
      }
docs: adds a set of 6 indices to the index array Corresponds to 2 triangles that make up a rectangle