scene_viewer: Make it possible to disable the animation feature (#4849)

# Objective

- The `scene_viewer` example assumes the `animation` feature is enabled, which it is by default. However, animations may have a performance cost that is undesirable when testing performance, for example. Then it is useful to be able to disable the `animation` feature and one would still like the `scene_viewer` example to work.

## Solution

- Gate animation code in `scene_viewer` on the `animation` feature being enabled.

examples/tools/scene_viewer.rs

@@ -41,8 +41,8 @@ Controls:
     Enter       - Cycle through animations
 "
     );
-    App::new()
-        .insert_resource(AmbientLight {
+    let mut app = App::new();
+    app.insert_resource(AmbientLight {
         color: Color::WHITE,
         brightness: 1.0 / 5.0f32,
     })
@@ -60,14 +60,18 @@ Controls:
     .add_system_to_stage(CoreStage::PreUpdate, camera_spawn_check)
     .add_system(update_lights)
     .add_system(camera_controller)
-        .add_system(start_animation)
-        .add_system(keyboard_animation_control)
-        .add_system(keyboard_cameras_control)
-        .run();
+    .add_system(keyboard_cameras_control);
+
+    #[cfg(feature = "animation")]
+    app.add_system(start_animation)
+        .add_system(keyboard_animation_control);
+
+    app.run();
 }
 
 struct SceneHandle {
     handle: Handle<Gltf>,
+    #[cfg(feature = "animation")]
     animations: Vec<Handle<AnimationClip>>,
     instance_id: Option<InstanceId>,
     is_loaded: bool,
@@ -82,6 +86,7 @@ fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
     info!("Loading {}", scene_path);
     commands.insert_resource(SceneHandle {
         handle: asset_server.load(&scene_path),
+        #[cfg(feature = "animation")]
         animations: Vec::new(),
         instance_id: None,
         is_loaded: false,
@@ -117,6 +122,8 @@ fn scene_load_check(
                 scene_handle.instance_id =
                     Some(scene_spawner.spawn(gltf_scene_handle.clone_weak()));
 
+                #[cfg(feature = "animation")]
+                {
                     scene_handle.animations = gltf.animations.clone();
                     if !scene_handle.animations.is_empty() {
                         info!(
@@ -129,6 +136,7 @@ fn scene_load_check(
                             }
                         );
                     }
+                }
 
                 info!("Spawning scene...");
             }
@@ -143,6 +151,7 @@ fn scene_load_check(
     }
 }
 
+#[cfg(feature = "animation")]
 fn start_animation(
     mut player: Query<&mut AnimationPlayer>,
     mut done: Local<bool>,
@@ -158,6 +167,7 @@ fn start_animation(
     }
 }
 
+#[cfg(feature = "animation")]
 fn keyboard_animation_control(
     keyboard_input: Res<Input<KeyCode>>,
     mut animation_player: Query<&mut AnimationPlayer>,