The MKVToolNix GUI has been marked as unsupported on darwin since
88d7718a64 (mkvtoolnix-cli: fix build on darwin, 2017-03-13), but
currently it builds without issues. It is when mkvtoolnix-gui is run
that the window doesn't appear.
Setting the environment variable QT_MAC_WANTS_LAYER=1 fixes it and now
the GUI runs on Darwin too.
Tested on Big Sur.
Picked from 461466306e (octaveFull: set QT_MAC_WANTS_LAYER=1 on Darwin,
2021-07-22).
a299818b79