Gamescope integration

Gamescope integration could be a nice optional feature for this package.

[Gamescope](https://github.com/Plagman/gamescope) is a Wayland microcompositor, which is very useful for playing games. Whether the game itself is run in native Wayland isn't relevant (#96), Xwayland is supported. You can run Gamescope from within another Wayland compositor like Mutter (GNOME) or KWin (KDE).

This has the noticeable benefit of easier game resolution scaling on Wayland (among other things). Currently the only way to change the game resolution on Wayland is to change the resolution of your display. X11 allows apps to change the display resolution, but Wayland does not. So currently on Wayland if you have a high resolution display and not-powerful hardware, you have no easy way of running the game at a lower resolution for better performance. [To solve this Gamescope allows you to scale output as desired](https://github.com/Plagman/gamescope#examples).

How this could be done in Flatpak I'm not sure, but it seems possible. [There is already a Steam Flatpak repo for it](https://github.com/flathub/com.valvesoftware.Steam.Utility.gamescope).