* mkvmerge: By default mkvmerge keeps the aspect ratio information
in AVC/h.264 video bitstreams now (equivalent to specifying
“–engage keep_bitstream_ar_info” in earlier versions). A new
option “–engage remove_bitstream_ar_info” is available that
restores the previous behaviour.
现在默认参数已经是–engage keep_bitstream_ar_info，所以不需要再添加这个额外参数来保持码流内ar信息，相对的添加了一条—engage remove_bitstream_ar_info来去掉码流内ar信息。