Log Message:
mkvtoolnix: updated to 51.0.0

Version 51.0.0 "I Wish" 2020-10-04

New features and enhancements

* mkvmerge, mkvpropedit, MKVToolNix GUI: IETF BCP 47 language tags: added
  missing support for UN M.49 regions for which there are no corresponding ISO
  3166 alpha codes. This enables language tags such as `es-419` = Spanish in
  Latin America and the Caribbean.
* MKVToolNix GUI: IETF BCP 47 language tags: the text in the widget displaying
  the current language & the corresponding edit button is now displayed like a
  link (depending on the theme: different color & underlined), making it
  clearer that not just the edit button can be clicked but the text, too.

Bug fixes

* mkvmerge: AV1 parser: fixed mkvmerge crashing after uncaught exceptions due
  to certain data conditions. Found by fuzzying.
* mkvmerge: AV1 packetizer: the duration of frames wasn't set properly. When
  appending AV1 IVF or OBU files this meant that the last frame of the Nth
  file and the first frame of file N+1 had the same timestamp.
* mkvmerge: DTS reader: fixed handling of buffers with an odd length when
  byte-swapped DTS is detected so that mkvmerge doesn't abort with a failed
  assertion. Found by fuzzying.
* mkvmerge: h.264/AVC and h.265/HEVC elementary stream readers: mkvmerge will
  no longer claim to recognize data that looks like AVC or HEVC but with
  invalid video width/height values as that lead to failed assertions in
  libEBML later. Found by fuzzying.
* mkvmerge: h.265/HEVC parser: fixed mkvmerge aborting after uncaught
  exceptions due to certain data constellations found by fuzzying.
* mkvmerge: IVF reader: fixed mkvmerge crashing with a "division by zero"
  error due to certain data conditions. Found by fuzzying.
* mkvmerge: fixed mkvmerge aborting with a "division by zero" error \ 
due to the
  timestamp scaling factor being 0 after handling certain data constellations
  found by fuzzying.
* mkvmerge: track statistics: fixed a "division by zero" error when the
  content's shorter than 1ms. Found by fuzzying.
* mkvmerge, mkvinfo, mkvextract, mkvpropedit: Matroska access class: fixed an
  invalid memory access under certain data conditions. Found by fuzzying.
* MKVToolNix GIU: IETF BCP 47/RFC 5646 language tags: the GUI will no longer
  open the language dialog when clicking on a disabled language display
  widget.
* MKVToolNix GUI: IETF BCP 47 language tags: the "variants" \ 
combo-boxes were
  not populated even when the language tag was valid and contained at a
  variant.
* MKVToolNix GUI: IETF BCP 47 language tags: when no language is selected, at
  least one of the other components (extended subtags, region, or variants)
  has something selected and "private use" is not empty, the GUI would \ 
claim
  this to be a valid tag, which it isn't.
* MKVToolNix GUI: multiplexer: when the tracks/chapters/tags selection changes
  from "at least one entry selected" to "no entry selected", \ 
the input
  controls will be reset to their default state and not just disabled.

Build system changes

* `configure` now checks for the presence of the a system-wide installed copy
  of the JPCRE2 C++ wrapper library for the PCRE2 library and uses that if
  it's new enough (at least v10.32.1). If not, the bundled version will be
  used as a fallback.