Log Message:
mkvtoolnix: updated to 57.0.0

Version 57.0.0 "Till The End" 2021-05-22

New features and enhancements

* mkvmerge: MP4 reader: added support for reading Dolby Vision from MP4 files
  (FourCCs `dvh1` and `dvhe`; configuration records `dvcC`, `dvvC` and `hvcE`
  will be converted into block addition mappings).
* mkvmerge: SRT subtitles: mkvmerge now accepts empty text files with the
  extension `.srt` as SRT subtitle files, enabling the creation of empty SRT
  tracks.
* mkvmerge, mkvextract: HEVC/H.265: access unit delimiter NALUs will no longer
  be discarded, neither during muxing nor during extraction.
* MKVToolNix GUI: preferences: switched the order & wording of controls in the
  "enabling items" panel to make it clearer that certain controls define
  exceptions.

Bug fixes

* mkvmerge: HEVC/H.265 parser: several NALU types, notably the Dolby
  Vision-specific NALUs ("unspecified 62" and "unspecified \ 
63") and suffix SEI
  NALUs, are now stored with the frame they belong to instead of with the next
  frame.
* mkvmerge: HEVC/H.265 packetizer: fixed setting the track's default duration
  when reading HEVC/H.265 from Matroska files that don't have a default
  duration set.
* mkvmerge: HEVC/H.265 packetizer: fixed the calculation of the duration of
  frames so that `SimpleBlock` elements can be used again instead of
  `BlockGroups` with `BlockDuration` elements.
* mkvmerge, mkvextract: HEVC/H.265 parser: fixed issues with ordering &
  duplication of certain NALUs (parameter set & prefix SEI NALUs).
* MKVToolNix GUI: multiplexer: when dragging & dropping files to the
  multiplexer, the source directory will be remembered as the "last open
  directory" again, causing subsequent uses of the "open file" \ 
dialog to start
  in the same directory.
* mkvinfo: fixed the position of frames in block groups containing "block
  duration" elements in summary mode (e.g. for subtitle tracks).

Build system changes

* `configure` will now try to detect `libmagic` via `pkg-config` and fall back
  to including & linking directly if it cannot be found via `pkg-config`.