Directional lights cascade shadows not working when using multiple render layers

## Bevy version

0.15-rc.3

## Relevant system information

```
AdapterInfo { name: "Apple M2 Max", vendor: 0, device: 0, device_type: IntegratedGpu, driver: "", driver_info: "", backend: Metal }
```

## What you did

I've up to four viewports in my app, for each viewport I'm instantiating a ground, a directional light, a camera and 2x GLFT models. Each entity in the viewport has a render layer assigned. For GLTF models I [add render layers to all the scene meshes after instantiation following this hack](https://github.com/bevyengine/bevy/issues/12461#issuecomment-2439874485).

See the code: https://gist.github.com/doup/161a8269d78762bd16c2d3c1e5f5999a

## What went wrong

The shadows only appear in one of the viewports. Which viewport gets the shadows is random for each instantiation (is not changing every frame).


## Additional information

The same setup using point lights works fine:


I suppose `CascadeShadows` work only for one render layer?

Start with the linked gist and the CascadeShadows setup in Bevy 0.15-rc.3, reproducing the four-viewport configuration with separate render layers on the Metal adapter. Compare the directional-light shadow behavior with the working point-light setup; done means cascaded shadows appear correctly in every viewport rather than only one.

Written by the indexing model from the issue text.