jp2_output_box::write_header_last

This function is provided to facilitate the generation of very long boxes, where the box length cannot be determined in advance, and we do not wish to buffer the box's contents. Currently, an error will be generated unless the underlying jp2_family_tgt object is a seekable file, since seeking is required to write the box header after the box's contents have been written. For this reason, you should not use this function unless absolutely necessary. The mj2_target object uses it to write movies, since the entire movie must be written first, before the length can be determined to write in the box header.