As a prerequisite for decals and clustering of light probes, we want
clustering to operate on objects other than lights. (Currently, it only
operates on point and spot lights.) This necessitates a large
refactoring, so I'm splitting it up into small steps.
The first such step is to separate clustering from lighting by moving
clustering-related types and functions out of lighting and into their
own module subtree within the `bevy_pbr` crate. (Ultimately, we may want
to move it to `bevy_render`, but that requires more work and can be a
followup.)
No code changes have been made other than adjusting import lists and
moving code. This is to make this code easy to review. Ultimately, I
want to rename "light" to "clusterable object" in most cases, but doing
that at the same time as moving the code would make reviewing harder. So
instead I'm moving the code first and will follow this up with renaming.
## Migration Guide
* Clustering-related types and functions (e.g.
`assign_lights_to_clusters`) have moved under `bevy_pbr::cluster`, in
preparation for the ability to cluster objects other than lights.
5c74c17c24