Part of #13681
# Objective
gLTF Assets shouldn't be duplicated between Assets resource and node
children.
Also changed `asset_label` to be a method as [per previous PR
comment](https://github.com/bevyengine/bevy/pull/13558).
## Solution
- Made GltfNode children be Handles instead of asset copies.
## Testing
- Added tests that actually test loading and hierarchy as previous ones
unit tested only one function and that makes little sense.
- Made circular nodes an actual loading failure instead of a warning
no-op. You [_MUST NOT_ have cycles in
gLTF](https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#nodes-and-hierarchy)
according to the spec.
- IMO this is a bugfix, not a breaking change. But in an extremely
unlikely event in which you relied on invalid behavior for loading gLTF
with cyclic children, you will not be able to do that anymore. You
should fix your gLTF file as it's not valid according to gLTF spec. For
it to for work someone, it had to be bevy with bevy_animation flag off.
---
## Changelog
### Changed
- `GltfNode.children` are now `Vec<Handle<GltfNode>>` instead of
`Vec<GltfNode>`
- Having children cycles between gLTF nodes in a gLTF document is now an
explicit asset loading failure.
## Migration Guide
If accessing children, use `Assets<GltfNode>` resource to get the actual
child object.
#### Before
```rs
fn gltf_print_first_node_children_system(gltf_component_query: Query<Handle<Gltf>>, gltf_assets: Res<Assets<Gltf>>, gltf_nodes: Res<Assets<GltfNode>>) {
for gltf_handle in gltf_component_query.iter() {
let gltf_root = gltf_assets.get(gltf_handle).unwrap();
let first_node_handle = gltf_root.nodes.get(0).unwrap();
let first_node = gltf_nodes.get(first_node_handle).unwrap();
let first_child = first_node.children.get(0).unwrap();
println!("First nodes child node name is {:?)", first_child.name);
}
}
```
#### After
```rs
fn gltf_print_first_node_children_system(gltf_component_query: Query<Handle<Gltf>>, gltf_assets: Res<Assets<Gltf>>, gltf_nodes: Res<Assets<GltfNode>>) {
for gltf_handle in gltf_component_query.iter() {
let gltf_root = gltf_assets.get(gltf_handle).unwrap();
let first_node_handle = gltf_root.nodes.get(0).unwrap();
let first_node = gltf_nodes.get(first_node_handle).unwrap();
let first_child_handle = first_node.children.get(0).unwrap();
let first_child = gltf_nodes.get(first_child_handle).unwrap();
println!("First nodes child node name is {:?)", first_child.name);
}
}
```
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