x11: add nested x11 backend

src/backend/mod.rs

@@ -0,0 +1,18 @@
+// SPDX-License-Identifier: GPL-3.0-only
+
+use crate::state::State;
+use anyhow::Result;
+use smithay::reexports::calloop::EventLoop;
+
+pub mod x11;
+// TODO
+// pub mod wayland; // tbd in smithay
+// pub mod udev;
+
+pub fn init_backend_auto(event_loop: &mut EventLoop<State>, state: &mut State) -> Result<()> {
+    if std::env::var_os("DISPLAY").is_some() {
+        x11::init_backend(event_loop, state)
+    } else {
+        unimplemented!("Currently this runs only nested")
+    }
+}