Fix panic with multiple fog volumes (#18119)

# Objective

Fixes #17590.

## Solution

`prepare_volumetric_fog_uniforms` adds a uniform for each combination of
fog volume and view. But it only allocated enough uniforms for one fog
volume per view.

## Testing

Ran the `volumetric_fog` example with 1/2/3/4 fog volumes. Also checked
the `fog_volumes` and `scrolling_fog` examples (without multiple
volumes). Win10/Vulkan/Nvidia.

To test multiple views I tried adding fog volumes to the `split_screen`
example. This doesn't quite work - the fog should be centred on the fox,
but instead it's centred on the window. Same result with and without the
PR, so I'm assuming it's a separate bug.


![image](https://github.com/user-attachments/assets/4a74d6d7-8a73-4322-9dc6-c5ddd054ece2)

crates/bevy_pbr/src/volumetric_fog/render.rs

@@ -694,20 +694,20 @@ pub fn prepare_volumetric_fog_uniforms(
     render_queue: Res<RenderQueue>,
     mut local_from_world_matrices: Local<Vec<Mat4>>,
 ) {
-    let Some(mut writer) = volumetric_lighting_uniform_buffer.get_writer(
-        view_targets.iter().len(),
-        &render_device,
-        &render_queue,
-    ) else {
-        return;
-    };
-
     // Do this up front to avoid O(n^2) matrix inversion.
     local_from_world_matrices.clear();
     for (_, _, fog_transform) in fog_volumes.iter() {
         local_from_world_matrices.push(fog_transform.compute_matrix().inverse());
     }
 
+    let uniform_count = view_targets.iter().len() * local_from_world_matrices.len();
+
+    let Some(mut writer) =
+        volumetric_lighting_uniform_buffer.get_writer(uniform_count, &render_device, &render_queue)
+    else {
+        return;
+    };
+
     for (view_entity, extracted_view, volumetric_fog) in view_targets.iter() {
         let world_from_view = extracted_view.world_from_view.compute_matrix();