Implement PBR anisotropy per KHR_materials_anisotropy. (bevyengine#13450)

This commit implements support for physically-based anisotropy in Bevy's
`StandardMaterial`, following the specification for the
[`KHR_materials_anisotropy`] glTF extension.

[*Anisotropy*] (not to be confused with [anisotropic filtering]) is a
PBR feature that allows roughness to vary along the tangent and
bitangent directions of a mesh. In effect, this causes the specular
light to stretch out into lines instead of a round lobe. This is useful
for modeling brushed metal, hair, and similar surfaces. Support for
anisotropy is a common feature in major game and graphics engines;
Unity, Unreal, Godot, three.js, and Blender all support it to varying
degrees.

Two new parameters have been added to `StandardMaterial`:
`anisotropy_strength` and `anisotropy_rotation`. Anisotropy strength,
which ranges from 0 to 1, represents how much the roughness differs
between the tangent and the bitangent of the mesh. In effect, it
controls how stretched the specular highlight is. Anisotropy rotation
allows the roughness direction to differ from the tangent of the model.

In addition to these two fixed parameters, an *anisotropy texture* can
be supplied. Such a texture should be a 3-channel RGB texture, where the
red and green values specify a direction vector using the same
conventions as a normal map ([0, 1] color values map to [-1, 1] vector
values), and the the blue value represents the strength. This matches
the format that the [`KHR_materials_anisotropy`] specification requires.
Such textures should be loaded as linear and not sRGB. Note that this
texture does consume one additional texture binding in the standard
material shader.

The glTF loader has been updated to properly parse the
`KHR_materials_anisotropy` extension.

A new example, `anisotropy`, has been added. This example loads and
displays the barn lamp example from the [`glTF-Sample-Assets`]
repository. Note that the textures were rather large, so I shrunk them
down and converted them to a mixture of JPEG and KTX2 format, in the
interests of saving space in the Bevy repository.

[*Anisotropy*]:
https://google.github.io/filament/Filament.md.html#materialsystem/anisotropicmodel

[anisotropic filtering]:
https://en.wikipedia.org/wiki/Anisotropic_filtering

[`KHR_materials_anisotropy`]:
https://github.com/KhronosGroup/glTF/blob/main/extensions/2.0/Khronos/KHR_materials_anisotropy/README.md

[`glTF-Sample-Assets`]:
https://github.com/KhronosGroup/glTF-Sample-Assets/

* Physically-based anisotropy is now available for materials, which
enhances the look of surfaces such as brushed metal or hair. glTF scenes
can use the new feature with the `KHR_materials_anisotropy` extension.

With anisotropy:
![Screenshot 2024-05-20
233414](https://github.com/bevyengine/bevy/assets/157897/379f1e42-24e9-40b6-a430-f7d1479d0335)

Without anisotropy:
![Screenshot 2024-05-20
233420](https://github.com/bevyengine/bevy/assets/157897/aa220f05-b8e7-417c-9671-b242d4bf9fc4)

Author: pcwalton

crates/bevy_pbr/src/render/pbr_functions.wgsl

@@ -181,6 +181,7 @@ fn apply_normal_mapping(
     double_sided: bool,
     is_front: bool,
     in_Nt: vec3<f32>,
+    mip_bias: f32,
 ) -> vec3<f32> {
     // Unpack the TBN vectors.
     var T = TBN[0];

examples/3d/anisotropy.rs

@@ -78,14 +78,14 @@ fn setup(mut commands: Commands, asset_server: Res<AssetServer>, app_status: Res
         ..default()
     });
 
-    spawn_text(&mut commands, &app_status);
+    spawn_text(&mut commands, &asset_server, &app_status);
 }
 
 /// Spawns the help text.
-fn spawn_text(commands: &mut Commands, app_status: &AppStatus) {
+fn spawn_text(commands: &mut Commands, asset_server: &AssetServer, app_status: &AppStatus) {
     commands.spawn(
         TextBundle {
-            text: app_status.create_help_text(),
+            text: app_status.create_help_text(asset_server),
             ..default()
         }
         .with_style(Style {
@@ -223,9 +223,19 @@ fn handle_input(
 }
 
 /// A system that updates the help text based on the current app status.
+<<<<<<< HEAD
 fn update_help_text(mut text_query: Query<&mut Text>, app_status: Res<AppStatus>) {
     for mut text in text_query.iter_mut() {
         *text = app_status.create_help_text();
+=======
+fn update_help_text(
+    mut text_query: Query<&mut Text>,
+    app_status: Res<AppStatus>,
+    asset_server: Res<AssetServer>,
+) {
+    for mut text in text_query.iter_mut() {
+        *text = app_status.create_help_text(&asset_server);
+>>>>>>> ef156ebb0 (Implement PBR anisotropy per `KHR_materials_anisotropy`. (#13450))
     }
 }
 
@@ -274,7 +284,11 @@ fn spawn_point_light(commands: &mut Commands) {
 
 impl AppStatus {
     /// Creates the help text as appropriate for the current app status.
+<<<<<<< HEAD
     fn create_help_text(&self) -> Text {
+=======
+    fn create_help_text(&self, asset_server: &AssetServer) -> Text {
+>>>>>>> ef156ebb0 (Implement PBR anisotropy per `KHR_materials_anisotropy`. (#13450))
         // Choose the appropriate help text for the anisotropy toggle.
         let material_variant_help_text = if self.anisotropy_enabled {
             "Press Enter to disable anisotropy"
@@ -292,7 +306,15 @@ impl AppStatus {
         // Build the `Text` object.
         Text::from_section(
             format!("{}\n{}", material_variant_help_text, light_help_text),
+<<<<<<< HEAD
             TextStyle::default(),
+=======
+            TextStyle {
+                font: asset_server.load("fonts/FiraMono-Medium.ttf"),
+                font_size: 24.0,
+                ..default()
+            },
+>>>>>>> ef156ebb0 (Implement PBR anisotropy per `KHR_materials_anisotropy`. (#13450))
         )
     }
 }