[Feature] Local / split screen multiplayer support

[jroyvde]

I think it would be wonderful for GZDoom to be able to have a couch multiplayer session with friends, playing the base games like Doom, Heretic and Hexen, or any of the huge variety of multiplayer focused mods available. This feature is already available in the official Doom + Doom II re-release. It'd be great to eventually have something similar implemented in GZDoom.

[RicardoLuis0]

if you have two controllers, that can already be done by running a "local" multiplayer session, and making the game run on the background with i_pauseinbackground false i_soundinbackground true, enable only one controller per game in the controller config, and enable controller input in background