Be able to get the Backend from the Compositor

[Timidger]

Depending on the backend you may be able to do certain things (e.g. fake outputs on the Wayland/X11 backends).

To accomplish this, we should have a Backend enum that is returned from the compositor.

It should look something like the following:

enum Backend {
    Wayland(*mut wlr_backend),
    X11(*mut wlr_backend),
    DRM(*mut wlr_backend),
    Headless(*mut wlr_backend),
    Multi(Vec<Backend>)
}

Related to #169.