move calculate_cluster_factors to cluster assign (#19958)

# Objective

- Make bevy_light possible by making it possible to split out
clusterable into bevy_camera

## Solution

- Move some stuff so i can split it out cleanly.

## Testing

- 3d_scene runs

crates/bevy_pbr/src/cluster/assign.rs

@@ -381,7 +381,7 @@ pub(crate) fn assign_objects_to_clusters(
 
         // NOTE: Ensure the far_z is at least as far as the first_depth_slice to avoid clustering problems.
         let far_z = far_z.max(first_slice_depth);
-        let cluster_factors = crate::calculate_cluster_factors(
+        let cluster_factors = calculate_cluster_factors(
             first_slice_depth,
             far_z,
             requested_cluster_dimensions.z as f32,
@@ -871,6 +871,23 @@ pub(crate) fn assign_objects_to_clusters(
     }
 }
 
+pub fn calculate_cluster_factors(
+    near: f32,
+    far: f32,
+    z_slices: f32,
+    is_orthographic: bool,
+) -> Vec2 {
+    if is_orthographic {
+        Vec2::new(-near, z_slices / (-far - -near))
+    } else {
+        let z_slices_of_ln_zfar_over_znear = (z_slices - 1.0) / ops::ln(far / near);
+        Vec2::new(
+            z_slices_of_ln_zfar_over_znear,
+            ops::ln(near) * z_slices_of_ln_zfar_over_znear,
+        )
+    }
+}
+
 fn compute_aabb_for_cluster(
     z_near: f32,
     z_far: f32,

crates/bevy_pbr/src/render/light.rs

@@ -1,4 +1,5 @@
 use self::assign::ClusterableObjectType;
+use crate::assign::calculate_cluster_factors;
 use crate::*;
 use bevy_asset::UntypedAssetId;
 use bevy_color::ColorToComponents;
@@ -11,7 +12,7 @@ use bevy_ecs::{
     prelude::*,
     system::lifetimeless::Read,
 };
-use bevy_math::{ops, Mat4, UVec4, Vec2, Vec3, Vec3Swizzles, Vec4, Vec4Swizzles};
+use bevy_math::{ops, Mat4, UVec4, Vec3, Vec3Swizzles, Vec4, Vec4Swizzles};
 use bevy_platform::collections::{HashMap, HashSet};
 use bevy_platform::hash::FixedHasher;
 use bevy_render::erased_render_asset::ErasedRenderAssets;
@@ -694,22 +695,6 @@ pub enum LightEntity {
         light_entity: Entity,
     },
 }
-pub fn calculate_cluster_factors(
-    near: f32,
-    far: f32,
-    z_slices: f32,
-    is_orthographic: bool,
-) -> Vec2 {
-    if is_orthographic {
-        Vec2::new(-near, z_slices / (-far - -near))
-    } else {
-        let z_slices_of_ln_zfar_over_znear = (z_slices - 1.0) / ops::ln(far / near);
-        Vec2::new(
-            z_slices_of_ln_zfar_over_znear,
-            ops::ln(near) * z_slices_of_ln_zfar_over_znear,
-        )
-    }
-}
 
 pub fn prepare_lights(
     mut commands: Commands,