Parser::parse_ftyp()

  public function parse_ftyp() {
    $box    = new Box();
    $status = $box->parse( $this->handle, $this->num_parsed_boxes );
    if ( $status != FOUND ) {
      return false;
    }

    if ( $box->type != 'ftyp' ) {
      return false;
    }
    // Iterate over brands. See ISO/IEC 14496-12:2012(E) 4.3.1
    if ( $box->content_size < 8 ) {
      return false;
    }
    for ( $i = 0; $i + 4 <= $box->content_size; $i += 4 ) {
      if ( !( $data = read( $this->handle, 4 ) ) ) {
        return false;
      }
      if ( $i == 4 ) {
        continue; // Skip minor_version.
      }
      if ( substr( $data, 0, 4 ) == 'avif' || substr( $data, 0, 4 ) == 'avis' ) {
        return skip( $this->handle, $box->content_size - ( $i + 4 ) );
      }
      if ( $i > 32 * 4 ) {
        return false; // Be reasonable.
      }

    }
    return false; // No AVIF brand no good.
  }