Mock spawn camera

This doesn't do anything useful. Commit just kept around in case I need
to revisit this.

`_setup_camera` commented out for the moment because we observed the
error mentioned in the comment
(bevyengine/bevy#11734).
With `do_nothing`, we get this:

```
2024-03-28T11:58:43.553731Z ERROR log: X11 error: XError {
    description: "BadWindow (invalid Window parameter)",
    error_code: 3,
    request_code: 148,
    minor_code: 1,
}
2024-03-28T11:58:43.567517Z ERROR bevy_winit: winit event loop returned an error: os error at /home/alex/.cargo/registry/src/index.crates.io-6f17d22bba15001f/winit-0.29.15/src/platform_impl/linux/x11/mod.rs:427: Xlib error: X error: BadWindow (invalid Window parameter) (code: 3, request code: 148, minor code: 1)
```

src/main.rs

@@ -2,6 +2,21 @@ use bevy::prelude::*;
 
 fn main() {
     App::new()
+        // this crashes when closing the window with https://github.com/bevyengine/bevy/issues/11734
+        // .add_systems(Startup, setup_camera)
+        // this does NOT crash, instead has this: https://gist.github.com/Ablesius/f8ed4b0a7252e49d1a4380189cae19eb
+        // try: replaced call to do_nothing with ()
+        .add_systems(Startup, do_nothing)
         .add_plugins(DefaultPlugins)
         .run();
 }
+
+fn _setup_camera(mut commands: Commands) {
+    // need to deviate from the blog entry
+    // based on https://github.com/marcusbuffett/bevy_snake/commit/d7f263735dd8cc7ef5d94b961f1f7554ece780a3#r113302872
+    commands.spawn(Camera2dBundle::default());
+}
+
+// TODO: drop-in
+// remove as soon as _setup_camera works
+fn do_nothing() {}