Multimap Support

[qxdFox]

primary use for me would be to have some sort of casino loaded on all servers instead of editing existing maps

[qxdFox]

I'll get back to this if ddnet#11658 gets merged

[AssassinTee]

This is already possible, see https://github.com/AssassinTee/MultiMap/tree/dev for example.

Watch-out for inter-dimensional hooking