Split out spot_light_world_from_view into a function in shadows.wgsl (#20050)

# Objective

- Improve readability

## Solution

- Split a function out

## Testing

- spotlight example works

Author: atlv24

crates/bevy_pbr/src/render/shadows.wgsl

@@ -62,6 +62,22 @@ fn fetch_point_shadow(light_id: u32, frag_position: vec4<f32>, surface_normal: v
     return sample_shadow_cubemap(frag_ls * flip_z, distance_to_light, depth, light_id);
 }
 
+// this method of constructing a basis from a vec3 is used by glam::Vec3::any_orthonormal_pair
+// so we reproduce it here to avoid a mismatch if glam changes. we also switch the handedness
+// the construction of the orthonormal basis up and right vectors needs to precisely mirror the code
+// in bevy_light/spot_light.rs:spot_light_world_from_view
+fn spot_light_world_from_view(fwd: vec3<f32>) -> mat3x3<f32> {
+    var sign = -1.0;
+    if (fwd.z >= 0.0) {
+        sign = 1.0;
+    }
+    let a = -1.0 / (fwd.z + sign);
+    let b = fwd.x * fwd.y * a;
+    let up_dir = vec3<f32>(1.0 + sign * fwd.x * fwd.x * a, sign * b, -sign * fwd.x);
+    let right_dir = vec3<f32>(-b, -sign - fwd.y * fwd.y * a, fwd.y);
+    return mat3x3<f32>(right_dir, up_dir, fwd);
+}
+
 fn fetch_spot_shadow(
     light_id: u32,
     frag_position: vec4<f32>,
@@ -88,17 +104,7 @@ fn fetch_spot_shadow(
         + ((*light).shadow_depth_bias * normalize(surface_to_light))
         + (surface_normal.xyz * (*light).shadow_normal_bias) * distance_to_light;
 
-    // the construction of the up and right vectors needs to precisely mirror the code
-    // in render/light.rs:spot_light_view_matrix
-    var sign = -1.0;
-    if (fwd.z >= 0.0) {
-        sign = 1.0;
-    }
-    let a = -1.0 / (fwd.z + sign);
-    let b = fwd.x * fwd.y * a;
-    let up_dir = vec3<f32>(1.0 + sign * fwd.x * fwd.x * a, sign * b, -sign * fwd.x);
-    let right_dir = vec3<f32>(-b, -sign - fwd.y * fwd.y * a, fwd.y);
-    let light_inv_rot = mat3x3<f32>(right_dir, up_dir, fwd);
+    let light_inv_rot = spot_light_world_from_view(fwd);
 
     // because the matrix is a pure rotation matrix, the inverse is just the transpose, and to calculate
     // the product of the transpose with a vector we can just post-multiply instead of pre-multiplying.