docs: protected ParsableBox createMvex(Movie movie) {
          MovieExtendsBox mvex = new MovieExtendsBox();
          final MovieExtendsHeaderBox mved = new MovieExtendsHeaderBox();
          mved.setVersion(1);
          for (Track track : movie.getTracks()) {
              final long trackDuration = getTrackDuration(movie, track);
              if (mved.getFragmentDuration() < trackDuration) {
                  mved.setFragmentDuration(trackDuration);
              }
          }
          mvex.addBox(mved);
  
          for (Track track : movie.getTracks()) {
              mvex.addBox(createTrex(movie, track));
          }
          return mvex;
      }