Fix potential out-of-bounds access in pbr_functions.wgsl (#12585)

# Objective

- Fix a potential out-of-bounds access in the `pbr_functions.wgsl`
shader.

## Solution

- Correctly compute the `GpuLights::directional_lights` array length.

## Comments

I think this solves this comment in the code, but need someone to test
it:
```rust
//NOTE: When running bevy on Adreno GPU chipsets in WebGL, any value above 1 will result in a crash
// when loading the wgsl "pbr_functions.wgsl" in the function apply_fog.
```

crates/bevy_pbr/src/render/light.rs

@@ -991,7 +991,8 @@ pub fn prepare_lights(
                 cluster_factors_zw.y,
             ),
             cluster_dimensions: clusters.dimensions.extend(n_clusters),
-            n_directional_lights: directional_lights.iter().len() as u32,
+            n_directional_lights: directional_lights.iter().len().min(MAX_DIRECTIONAL_LIGHTS)
+                as u32,
             // spotlight shadow maps are stored in the directional light array, starting at num_directional_cascades_enabled.
             // the spot lights themselves start in the light array at point_light_count. so to go from light
             // index to shadow map index, we need to subtract point light count and add directional shadowmap count.