[Feature Request] Allow `fps_limit` limit to be overriden by `MANGOHUD_FPS_LIMIT⁩` environment variable.

At the moment the `fps_limit` option in MangoHud.conf seems to override the `MANGOHUD_FPS_LIMIT⁩` environment variable.

I think it would be more useful, if it was the other way around, so that the user has a convenient way to override the FPS limit on a per-game basis, without having to create a custom MangoHud.conf file for every game that requires a different FPS limit.