code: function(rad, axis) {
          mat4.rotate(this.array, this.array, rad, axis.array);
          this._dirty = true;
          return this;
      }
docs: Rotate self by rad about axis.
  Equal to right-multiply a rotaion matrix
  @param  {number}   rad
  @param  {clay.Vector3} axis
  @return {clay.Matrix4}