New DebugUI plugin

This plugin is meant to have some add-ons to interact with the
simulation to provide insight.

This first change adds a cursor under the mouse with the intention that
it's circle can be used as a reticule for targeting boids. Hovering over
a cluster and getting their CoM or velocities, for example.

src/debug_plugin.rs

@@ -0,0 +1,50 @@
+use bevy::{prelude::*, sprite::MaterialMesh2dBundle, window::PrimaryWindow};
+
+pub struct BoidsDebugPlugin;
+
+impl Plugin for BoidsDebugPlugin {
+    fn build(&self, app: &mut App) {
+        app.add_systems(Startup, setup)
+            .add_systems(FixedUpdate, update_cursor);
+    }
+}
+
+fn setup(
+    mut commands: Commands,
+    mut meshes: ResMut<Assets<Mesh>>,
+    mut materials: ResMut<Assets<ColorMaterial>>,
+) {
+    commands.spawn((
+        Cursor,
+        MaterialMesh2dBundle {
+            mesh: meshes.add(Annulus::new(9.5, 10.0)).into(),
+            material: materials.add(Color::srgb(0.0, 0.0, 0.0)),
+            ..default()
+        },
+    ));
+}
+
+#[derive(Component)]
+struct Cursor;
+
+fn update_cursor(
+    window: Query<&Window, With<PrimaryWindow>>,
+    camera: Query<(&Camera, &GlobalTransform)>,
+    mut cursor_query: Query<&mut Transform, With<Cursor>>,
+) {
+    // I'm trusting that only one thing has the `Cursor` component
+    // It's defined here in this module, so that *should* be the case...
+    let win = window.get_single().unwrap();
+    let (cam, cam_transform) = camera.get_single().unwrap();
+    // the cursor might not be on the window. Only adjust position when it is.
+    if let Some(cursor_pos_in_window) = win.cursor_position() {
+        // transform the window position into world space
+        // I'm trusting that this viewport actually displays this camera
+        // (or at least that the coordinates can map correctly)
+        let cursor_in_world = cam
+            .viewport_to_world_2d(cam_transform, cursor_pos_in_window)
+            .unwrap();
+        let mut cursor = cursor_query.get_single_mut().unwrap();
+        cursor.translation = cursor_in_world.extend(0.0);
+    }
+}

src/main.rs

@@ -1,12 +1,15 @@
 use bevy::prelude::*;
 
 mod birdoids_plugin;
+mod debug_plugin;
 
 use birdoids_plugin::BoidsPlugin;
+use debug_plugin::BoidsDebugPlugin;
 
 fn main() {
     App::new()
         .add_plugins(DefaultPlugins)
+        .add_plugins(BoidsDebugPlugin)
         .add_plugins(BoidsPlugin)
         .run();
 }