use const Vec2 in lights cluster and bounding box when possible (bevy…

…engine#4602)

- noticed a few Vec3 and Vec2 that could be const

- Declared them as const
- It seems to make a tiny improvement in example `many_light`, but given that the change is not complex at all it could still be worth it

crates/bevy_pbr/src/light.rs

@@ -687,6 +687,8 @@ fn compute_aabb_for_cluster(
 
     Aabb::from_min_max(cluster_min, cluster_max)
 }
+const NDC_MIN: Vec2 = const_vec2!([-1.0, -1.0]);
+const NDC_MAX: Vec2 = const_vec2!([1.0, 1.0]);
 
 // Sort point lights with shadows enabled first, then by a stable key so that the index
 // can be used to limit the number of point light shadows to render based on the device and