mkvtoolnix: updated to 43.0.0

Version 43.0.0 "The Quartermaster":

New features and enhancements
* MKVToolNix GUI: using one of the command line options `--info`,
  `--edit-headers` or `--edit-chapters` will make the GUI select the
  corresponding tool on startup even if no further file name is
  given.
* MKVToolNix GUI: multiplexer: added a setting in the preferences for changing
  the location of the MediaInfo executable for when the user accidentally
  selected the wrong one.

Bug fixes
* mkvmerge: AVC/h.264 parser: the order of the NALUs before each key frame was
  sometimes wrong: mkvmerge wrote SPS & PPS after SEI NALUs. Now SPS & PPS
  NALUs are always written before the other NALUs. Patch by Torsten
  Hauska.
* mkvmerge: AVC/h.264 parser: when additional SPS or PPS NALUs (with IDs that
  haven't been seen so far) are found mid-stream, mkvmerge will prepend all
  following key frames with all current valid SPS & PPS NALUs (just like when
  SPS & PPS NALUs are overwritten mid-stream).
* mkvmerge: MPEG TS reader: fixed detection of MPEG-1/2 video tracks
  consisting of very few frames.
* mkvmerge: Ogg Opus reader: fixed the handling of Ogg Opus streams that
  contain a just one Ogg page (`mkvmerge` aborted with an exception).
* MKVToolNix GUI: multiplexer: dragging & dropping XML files with chapters or
  tags to the GUI's window was broken in v42. Instead of adding the file names
  to the appropriate input boxes the GUI was running mkvmerge for file
  identification purposes which then failed.


(adam)