getBoxOffset_(parent, type) {
    let offset = 8;
    for (let i = 0; i < parent.boxes.length; i++) {
      if (parent.boxes[i].type === type) {
        return offset;
      }
      offset += parent.boxes[i].size;
    }
    return offset;
  }
This function returns the offset of the 1st byte of a child box within
a container box.

@param {ISOBox} parent
@param {string} type
@return {number}
@private