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