MKVToolNix (64-bit)

What's new in this version:

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.
- 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.

Fixed:
- Mkvmerge: fixed a crash when splitting by chapters is enabled but there are no chapters at all
- 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
- Mkvmerge: AVC ES parser: fixed mkvmerge aborting due to uncaught exception when encountering bad SPS data
- 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
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.

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.