Need to manually set thickness property when importing gltf scene

## Bevy version

0.12.1

## What you did

1. Create new principled BSDF material in Blender
2. Set transmission close to 1
3. Set IOR to something other than 1 to get some refraction going
4. Export gltf
5. Load the scene in Bevy using the following code:
```rust
commands.spawn(SceneBundle {
scene: asset_server.load("scene.glb#Scene0"),
..default()
});
```

## What went wrong

No refraction is seen when the scene is loaded in the Bevy app. Need to manually set the `thickness` property of the StandardMaterial to achieve a similar look to the Eevee renderer in Blender. The StandardMaterial documentation does not seem to include how to approach this exactly. But ideally it would work out of the box.


## Additional information

Let me know if a reproducer would help, wanted to post first in case I just missed some export setting in Blender.

Start with the reported Blender-to-glTF workflow, the SceneBundle loading example, and StandardMaterial's transmission, IOR, and thickness documentation. Reproduce the scene in Bevy 0.12.1 and compare the imported material with the Blender Eevee result; done means the imported scene provides the expected refraction without manually setting thickness, or the required export limitation is documented.

Written by the indexing model from the issue text.