add AnimationPlayer component only on scene roots that are also animation roots (#4417)

# Objective

- Fix #4416 
- The scene has two root nodes, with the second one being the animation root

## Solution

- Check all scene root nodes, and add the `AnimationPlayer` component to nodes that are also animation roots

crates/bevy_gltf/src/loader.rs

@@ -133,19 +133,21 @@ async fn load_gltf<'a, 'b>(
 
     #[cfg(feature = "bevy_animation")]
     let paths = {
-        let mut paths = HashMap::<usize, Vec<Name>>::new();
+        let mut paths = HashMap::<usize, (usize, Vec<Name>)>::new();
         for scene in gltf.scenes() {
             for node in scene.nodes() {
-                paths_recur(node, &[], &mut paths);
+                let root_index = node.index();
+                paths_recur(node, &[], &mut paths, root_index);
             }
         }
         paths
     };
 
     #[cfg(feature = "bevy_animation")]
-    let (animations, named_animations) = {
+    let (animations, named_animations, animation_roots) = {
         let mut animations = vec![];
         let mut named_animations = HashMap::default();
+        let mut animation_roots = HashSet::default();
         for animation in gltf.animations() {
             let mut animation_clip = AnimationClip::default();
             for channel in animation.channels() {
@@ -192,7 +194,8 @@ async fn load_gltf<'a, 'b>(
                     return Err(GltfError::MissingAnimationSampler(animation.index()));
                 };
 
-                if let Some(path) = paths.get(&node.index()) {
+                if let Some((root_index, path)) = paths.get(&node.index()) {
+                    animation_roots.insert(root_index);
                     animation_clip.add_curve_to_path(
                         EntityPath {
                             parts: path.clone(),
@@ -218,7 +221,7 @@ async fn load_gltf<'a, 'b>(
             }
             animations.push(handle);
         }
-        (animations, named_animations)
+        (animations, named_animations, animation_roots)
     };
 
     let mut meshes = vec![];
@@ -470,10 +473,16 @@ async fn load_gltf<'a, 'b>(
         }
 
         #[cfg(feature = "bevy_animation")]
-        if !animations.is_empty() {
-            world
-                .entity_mut(*node_index_to_entity_map.get(&0).unwrap())
-                .insert(AnimationPlayer::default());
+        {
+            // for each node root in a scene, check if it's the root of an animation
+            // if it is, add the AnimationPlayer component
+            for node in scene.nodes() {
+                if animation_roots.contains(&node.index()) {
+                    world
+                        .entity_mut(*node_index_to_entity_map.get(&node.index()).unwrap())
+                        .insert(AnimationPlayer::default());
+                }
+            }
         }
 
         for (&entity, &skin_index) in &entity_to_skin_index_map {
@@ -529,13 +538,18 @@ fn node_name(node: &Node) -> Name {
     Name::new(name)
 }
 
-fn paths_recur(node: Node, current_path: &[Name], paths: &mut HashMap<usize, Vec<Name>>) {
+fn paths_recur(
+    node: Node,
+    current_path: &[Name],
+    paths: &mut HashMap<usize, (usize, Vec<Name>)>,
+    root_index: usize,
+) {
     let mut path = current_path.to_owned();
     path.push(node_name(&node));
     for child in node.children() {
-        paths_recur(child, &path, paths);
+        paths_recur(child, &path, paths, root_index);
     }
-    paths.insert(node.index(), path);
+    paths.insert(node.index(), (root_index, path));
 }
 
 /// Loads a glTF texture as a bevy [`Image`] and returns it together with its label.