Fix black spots appearing due to NANs when SSAO is enabled (#8926)

# Objective

Fixes #8925

## Solution

~~Clamp the bad values.~~

Normalize the prepass normals when we get them in the `prepass_normal()`
function.

## More Info

The issue is that NdotV is sometimes very slightly greater than 1 (maybe
FP rounding issues?), which caused `F_Schlick()` to return NANs in
`pow(1.0 - NdotV, 5.0)` (call stack looked like`pbr()` ->
`directional_light()` -> `Fd_Burley()` -> `F_Schlick()`)

crates/bevy_pbr/src/prepass/prepass_utils.wgsl

@@ -20,7 +20,7 @@ fn prepass_normal(frag_coord: vec4<f32>, sample_index: u32) -> vec3<f32> {
 #else
  let normal_sample = textureLoad(view_bindings::normal_prepass_texture, vec2<i32>(frag_coord.xy), 0);
 #endif // MULTISAMPLED
- return normal_sample.xyz * 2.0 - vec3(1.0);
+ return normalize(normal_sample.xyz * 2.0 - vec3(1.0));
 }
 #endif // NORMAL_PREPASS