MKVToolNix 21.0.0 (64-bit)

What's new in this version:

- New features and enhancements:
- mkvmerge: track statistics tags: the `TagDefault` element will not be written anymore as it was always set to the default value `1` anyway
- mkvmerge, MKVToolNix GUI: JSON files can now contain C++-style line comments outside of strings (e.g. something like this: `// this is ignored`). Such comments, even though not part of the official JSON specifications, are now ignored when reading JSON files
- MKVToolNix GUI: chapter editor: opening a Matroska file without chapters in it will now open the file in an empty chapter editor instead of showing an error message
- MKVToolNix GUI: an "info" tool has been added, replacing the functionality of mkvinfo's GUI. The functionality is not on par yet but will be for release v22

Bug fixes:
- build system: `configure` was treating `--disable-ubsan` and `--disable-addrsan` the same as `--enable-ubsan` and `--enable-addrsan`
- build system: an error message is output if a command to execute is not found instead of silently failing
- build system: in addition to looking for the `gettext` C function and library, `configure` now also verifies the presence of the `msgfmt` program instead of simply relying on it
- mkvmerge: appending files with additional parts at the same time was broken if more than one additional part was appended (e.g. when appending files from DVDs with something like `'(' VTS_01_1.VOB VTS_01_2.VOB ')' + '(' VTS_02_1.VOB VTS_02_2.VOB ')'`). In such a situation the content from files `VTS_02_1.VOB` and `VTS_02_2.VOB` where laid out in parallel to the content from the earlier files
- mkvmerge: FLV reader: a single invalid AAC frame was written for AAC audio tracks with codec initialization data longer than five bytes
- mkvmerge: FLV reader: timestamps will be normalized down to 0. Fixes #2220.
- mkvmerge: MP4 reader: if an AAC track doesn't contain an AAC-specific decoder configuration in the ESDS portion, then a default decoder configuration will be generated based on the track's header data instead of skipping the track. Fixes #2221.
- mkvmerge: MP4 reader: fixed reading HEVC/h.265 video tracks if they're stored as Annex B byte streams inside MP4
- mkvmerge: Ogg Opus reader: mkvmerge will now emit a warning instead of aborting when it encounters an Ogg Opus page with no data in the packet
- mkvmerge, mkvextract: Matroska parser: fixed a segmentation fault that occurred whenever the first level 1 element after resyncing after an error in the file structure isn't a cluster
- mkvmerge, MKVToolNix GUI multiplexer & header editor: fixed a crash during file type detection for attachments if MKVToolNix is installed in a path with non-ASCII characters (e.g. German Umlauts)
- mkvinfo: the `--hex-positions` parameter did nothing in summary mode.
- mkvinfo: Windows: line endings will be written as `rn` (carriage return & line feed) again instead of just `n` (line feed)
- mkvpropedit: adding track statistics tags: for tracks with content encoding (compression) mkvpropedit is now accounting the uncompressed number of bytes, not the encoded (compressed) number of bytes
- MKVToolNix GUI: multiplexer: the subtitle character set can now be set for appended subtitle files, too
- MKVToolNix GUI: multiplexer: when appending, all tracks appended to disabled tracks will start out disabled, too

Build system changes:
- mkvinfo: the GUI portion has been removed. mkvinfo is now a pure command-line program again