no-camera many_buttons argument, only emit UI camera warnings once (#17557)

# Objective

* Add a `no-camera` argument to the `many_buttons` stress test example.
* Only emit the UI "no camera found" warnings once.

crates/bevy_ui/src/layout/mod.rs

@@ -12,6 +12,7 @@ use bevy_math::{UVec2, Vec2};
 use bevy_render::camera::{Camera, NormalizedRenderTarget};
 use bevy_sprite::BorderRect;
 use bevy_transform::components::Transform;
+use bevy_utils::once;
 use bevy_window::{PrimaryWindow, Window, WindowScaleFactorChanged};
 use thiserror::Error;
 use tracing::warn;
@@ -164,10 +165,10 @@ pub fn ui_layout_system(
             match camera_with_default(target_camera) {
                 Some(camera_entity) => {
                     let Ok((_, camera)) = cameras.get(camera_entity) else {
-                        warn!(
+                        once!(warn!(
                             "UiTargetCamera (of root UI node {entity}) is pointing to a camera {} which doesn't exist",
                             camera_entity
-                        );
+                        ));
                         return;
                     };
                     let layout_info = camera_layout_info
@@ -177,13 +178,13 @@ pub fn ui_layout_system(
                 }
                 None => {
                     if cameras.is_empty() {
-                        warn!("No camera found to render UI to. To fix this, add at least one camera to the scene.");
+                        once!(warn!("No camera found to render UI to. To fix this, add at least one camera to the scene."));
                     } else {
-                        warn!(
+                        once!(warn!(
                             "Multiple cameras found, causing UI target ambiguity. \
                             To fix this, add an explicit `UiTargetCamera` component to the root UI node {}",
                             entity
-                        );
+                        ));
                     }
                 }
             }

examples/stress_tests/many_buttons.rs

@@ -50,6 +50,10 @@ struct Args {
     /// set the root node to display none, removing all nodes from the layout.
     #[argh(switch)]
     display_none: bool,
+
+    /// spawn the layout without a camera
+    #[argh(switch)]
+    no_camera: bool,
 }
 
 /// This example shows what happens when there is a lot of buttons on screen.
@@ -82,9 +86,11 @@ fn main() {
     })
     .add_systems(Update, (button_system, set_text_colors_changed));
 
-    app.add_systems(Startup, |mut commands: Commands| {
-        commands.spawn(Camera2d);
-    });
+    if !args.no_camera {
+        app.add_systems(Startup, |mut commands: Commands| {
+            commands.spawn(Camera2d);
+        });
+    }
 
     if args.grid {
         app.add_systems(Startup, setup_grid);