To allow this to work in-game without making major changes to the config infrastructure, when the server sends the config packet, we'll save a copy of it. When the user changes their config while in-game, we make another copy of that packet, and use said copy to mix the server settings with the client's like on normal join.

Should allow Fabric users to enable/disable immersives without using the config file.

We also need to re-send the client-to-server config, but that shouldn't be difficult