mkvtoolnix: updated to 64.0.0

Version 64.0.0 "Willows" 2021-12-27

New features and enhancements

* MKVToolNix GUI: language dialog: when the user switches between the two edit
  modes, the mode's corresponding first control is automatically focussed.

Bug fixes

* all: language lists: the entries for "Ancient Greek (to 1453)" and "Modern
  Greek (1453-)" have been renamed to "Greek (ancient, -1453)" and "Greek
  (modern, 1453-)" respectively in order to be easier to find.
* mkvmerge: VobSub reader: files containing `id: --` lines to indicate that
  the track language isn't known were rejected by `mkvmerge`. Now they're
  accepted, and `und` (undetermined) is used as the language.
* MKVToolNix GUI: chapter editor: the "Save to Matroska or WebM file" function
  was using the wrong standard file dialog (the "open file" one instead of
  "save"), leading to confusing button names.
* MKVToolNix GUI: high DPI scaling: fixed the icons in the tool selector
  having the wrong size for scaling factors other than 100% or 200%.

Build system changes

* The bundled `fmt` library was updated to v8.0.1.
* The bundled `nlohmann-json` library was updated to v3.9.1.
* The bundled `pugixml` library was updated to v1.11 revision 6a76cac.
* The bundled `utf8-cpp` library was updated to v3.2 revision b85efd6.

(adam)