MKVToolNix v62.0.0

# Version 62.0.0 "Apollo" 2021-10-10

## New features and enhancements

* all: IETF BCP 47/TFC 5646 language tags: all ISO 639 languages are now
  available, even the ones that aren't of types "Constructed", "Living" or
  "Special".
* mkvmerge, mkvpropedit: chapters: both programs will now write elements set
  to their default value, too (e.g. the legacy language element if it's set to
  `eng`). This is done to be more consistent with how MKVToolNix GUI writes
  chapters. Implements #3210.
* MKVToolNix GUI: multiplexer: when adding new files as attachments the GUI
  checks if there are other attachments with the same name. If so, the GUI
  would ask whether to skip the affected files or add them anyway. There's now
  an option in the preferences to always skip such files, and it's enabled by
  default. Implements #3213.
* MKVToolNix GUI: chapter editor: removed the support for explicitly setting
  the legacy country elements. Instead the legacy country elements are derived
  from the region parts of the IETF BCP 47 languages. Part of the
  implementation of #3193.

## Bug fixes

* mkvmerge: fixed a crash when splitting by chapters is enabled but there are
  no chapters at all. Part of the fix of #3198.
* mkvmerge: the error message when trying to split by a chapter number that
  doesn't exist contained the wrong number of chapters that actually do
  exist. Part of the fix of #3198.
* mkvmerge: AVC ES parser: fixed mkvmerge aborting due to uncaught exception
  when encountering bad SPS data. Part of the fix of #3176.
* mkvmerge: AVC/H.264 parser: re-added the hack `--engage
  all_i_slices_are_key_frames` which was accidentally removed in release v61.
* mkvmerge: AVI reader: fixed a crash trying to allocate too big of a memory
  chunk due to an integer overflows in check conditions. Part of the fix of
  #3176.
* mkvmerge: HEVC ES parser: fixed another issue with frame type
  recognition. Certain frames were marked as B frames/discardable on the
  container level even though they could be reference pictures. Now only SLNR
  pictures (sub-layer non-reference) are marked that way. Fixes #3192.
* mkvmerge: HEVC ES reader: improved file content detection for HEVC ES files
  with wrong file name extensions (e.g. `.mkv`), which fixes them being
  mis-detected as something else, e.g. DTS. Fixes #3201.
* mkvmerge: HEVC ES parser: "end of sequence" NALUs are kept now, and no
  superfluous flushing will be done when one is found. This prevents splitting
  NALUs into two different Matroska blocks when they should really be part of
  a single one, also causing problems with erroneous timestamps. Mostly
  affects DoVi NALUs. Fixes #3202.
* mkvmerge: Matroska reader, TrueHD: mkvmerge will now probe much more TrueHD
  frames trying to find the first sync frame. This fixes track detection in
  situations when a TrueHD doesn't start with a sync frame, e.g. when the
  source file's the result of splitting between sync frames.
* mkvmerge, MKVToolNix GUI's chapter editor: IETF BCP 47/RFC 5646 language
  tags: when reading chapters from MPLS playlist files, the `ChapLanguageIETF`
  element will now be set to the configured default chapter language, not just
  the legacy `ChapterLanguage` element. Part of the fix of #3193.
* mkvmerge, mkvpropedit, MKVToolNix GUI's chapter editor: IETF BCP 47 elements
  will now always be created before writing chapters unless IETF BCP 47
  elements are disabled. Part of the fix of #3193.
* mkvmerge, mkvpropedit, MKVToolNix GUI's chapter editor: when a chapter
  display element contains legacy language & country elements but no IETF BCP
  47 elements and IETF BCP 47 elements aren't disabled, the IETF BCP 47
  elements created will contain the region from the legacy element. Part of
  the fix of #3193.
* mkvmerge, mkvpropedit, MKVToolNix GUI's chapter editor: Legacy country
  elements are now created when IETF BCP 47 elements are present & contain a
  region code allowed in legacy country elements. Part of the fix of #3193.

## Build system changes

* The `tools` sub-directory is now always built. The corresponding `configure`
  option `--with-tools` has been removed. The `install` target still doesn't
  install them, though.