Fix handling of double_sided for normal maps (bevyengine#10326)

# Objective

Right now, we flip the `world_normal` in response to `double_sided &&
!is_front`, however when calculating `N` from tangents and the normal
map, we don't flip the normal read from the normal map, which produces
extremely weird results.

## Solution

- Pass `double_sided` and `is_front` flags to the
`apply_normal_mapping()` function and use them to conditionally flip
`Nt`

## Comparison

Note: These are from a custom scene running with the `transmission`
branch, (bevyengine#8015) I noticed lighting got pretty weird for the back side of
translucent `double_sided` materials whenever I added a normal map.

### Before

<img width="1392" alt="Screenshot 2023-10-31 at 01 26 06"
src="https://github.com/bevyengine/bevy/assets/418473/d5f8c9c3-aca1-4c2f-854d-f0d0fd2fb19a">

### After

<img width="1392" alt="Screenshot 2023-10-31 at 01 25 42"
src="https://github.com/bevyengine/bevy/assets/418473/fa0e1aa2-19ad-4c27-bb08-37299d97971c">


---

## Changelog

- Fixed a bug where `StandardMaterial::double_sided` would interact
incorrectly with normal maps, producing broken results.

crates/bevy_pbr/src/render/pbr_fragment.wgsl

@@ -148,6 +148,8 @@ fn pbr_input_from_standard_material(
         pbr_input.N = pbr_functions::apply_normal_mapping(
             pbr_bindings::material.flags,
             pbr_input.world_normal,
+            double_sided,
+            is_front,
 #ifdef VERTEX_TANGENTS
 #ifdef STANDARDMATERIAL_NORMAL_MAP
             in.world_tangent,

crates/bevy_pbr/src/render/pbr_functions.wgsl

@@ -61,6 +61,8 @@ fn prepare_world_normal(
 fn apply_normal_mapping(
     standard_material_flags: u32,
     world_normal: vec3<f32>,
+    double_sided: bool,
+    is_front: bool,
 #ifdef VERTEX_TANGENTS
 #ifdef STANDARDMATERIAL_NORMAL_MAP
     world_tangent: vec4<f32>,
@@ -106,6 +108,11 @@ fn apply_normal_mapping(
     if (standard_material_flags & pbr_types::STANDARD_MATERIAL_FLAGS_FLIP_NORMAL_MAP_Y) != 0u {
         Nt.y = -Nt.y;
     }
+
+    if double_sided && !is_front {
+        Nt = -Nt;
+    }
+
     // NOTE: The mikktspace method of normal mapping applies maps the tangent-space normal from
     // the normal map texture in this way to be an EXACT inverse of how the normal map baker
     // calculates the normal maps so there is no error introduced. Do not change this code