mkvtoolnix: fix GUI on darwin (#133848)

The MKVToolNix GUI has been marked as unsupported on darwin since
88d7718a64d (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 461466306ef (octaveFull: set QT_MAC_WANTS_LAYER=1 on Darwin,
2021-07-22).

pkgs/applications/video/mkvtoolnix/default.nix

···
 
   dontWrapQtApps = true;
 
+  # Avoid Qt 5.12 problem on Big Sur: https://bugreports.qt.io/browse/QTBUG-87014
+  qtWrapperArgs = lib.optionals stdenv.isDarwin [
+    "--set QT_MAC_WANTS_LAYER 1"
+  ];
+
   postFixup = optionalString withGUI ''
     wrapQtApp $out/bin/mkvtoolnix-gui
   '';
···
     homepage = "https://mkvtoolnix.download/";
     license = licenses.gpl2Only;
     maintainers = with maintainers; [ codyopel rnhmjoj ];
-    platforms = platforms.linux
-      ++ optionals (!withGUI) platforms.darwin;
+    platforms = platforms.unix;
   };
 }