code: public Box boundsUnderRotation (Matrix3 matrix, Box result) {
          result.setToEmpty();
          for (Vector3 vertex : _vertices) {
              result.addLocal(matrix.transform(vertex, _vertex));
          }
          return result;
      }
docs: Computes the bounds of the frustum under the supplied rotation and places the results in
  the box provided.
  
  @return a reference to the result box, for chaining.