[Feature Request] Panning law

From Thomas Neumann (@neumatho):

I have a little request you may put on your todo list, namely support
of panning law, specially for FastTracker 2. I found this you may use
as reference:
https://forum.openmpt.org/index.php?topic=6084.0
http://weaselaudiolib.sourceforge.net/#panning_law
https://modarchive.org/forums/index.php?topic=3517.0
