Use WireframeColor to override global color (#13034)

# Objective

- The docs says the WireframeColor is supposed to override the default
global color but it doesn't.

## Solution

- Use WireframeColor to override global color like docs said it was
supposed to do.
- Updated the example to document this feature
- I also took the opportunity to clean up the code a bit

Fixes #13032

crates/bevy_pbr/src/wireframe.rs

@@ -64,6 +64,10 @@ pub struct Wireframe;
 /// it will still affect the color of the wireframe when [`WireframeConfig::global`] is set to true.
 ///
 /// This overrides the [`WireframeConfig::default_color`].
+//
+// TODO: consider caching materials based on this color.
+// This could blow up in size if people use random colored wireframes for each mesh.
+// It will also be important to remove unused materials from the cache.
 #[derive(Component, Debug, Clone, Default, Reflect)]
 #[reflect(Component, Default)]
 pub struct WireframeColor {
@@ -155,19 +159,12 @@ fn apply_wireframe_material(
         }
     }
 
-    let mut wireframes_to_spawn = vec![];
-    for (e, wireframe_color) in &wireframes {
-        let material = if let Some(wireframe_color) = wireframe_color {
-            materials.add(WireframeMaterial {
-                color: wireframe_color.color.into(),
-            })
-        } else {
-            // If there's no color specified we can use the global material since it's already set to use the default_color
-            global_material.handle.clone()
-        };
-        wireframes_to_spawn.push((e, material));
+    let mut material_to_spawn = vec![];
+    for (e, maybe_color) in &wireframes {
+        let material = get_wireframe_material(maybe_color, &mut materials, &global_material);
+        material_to_spawn.push((e, material));
     }
-    commands.insert_or_spawn_batch(wireframes_to_spawn);
+    commands.insert_or_spawn_batch(material_to_spawn);
 }
 
 type WireframeFilter = (With<Handle<Mesh>>, Without<Wireframe>, Without<NoWireframe>);
@@ -176,16 +173,21 @@ type WireframeFilter = (With<Handle<Mesh>>, Without<Wireframe>, Without<NoWirefr
 fn apply_global_wireframe_material(
     mut commands: Commands,
     config: Res<WireframeConfig>,
-    meshes_without_material: Query<Entity, (WireframeFilter, Without<Handle<WireframeMaterial>>)>,
+    meshes_without_material: Query<
+        (Entity, Option<&WireframeColor>),
+        (WireframeFilter, Without<Handle<WireframeMaterial>>),
+    >,
     meshes_with_global_material: Query<Entity, (WireframeFilter, With<Handle<WireframeMaterial>>)>,
     global_material: Res<GlobalWireframeMaterial>,
+    mut materials: ResMut<Assets<WireframeMaterial>>,
 ) {
     if config.global {
         let mut material_to_spawn = vec![];
-        for e in &meshes_without_material {
+        for (e, maybe_color) in &meshes_without_material {
+            let material = get_wireframe_material(maybe_color, &mut materials, &global_material);
             // We only add the material handle but not the Wireframe component
             // This makes it easy to detect which mesh is using the global material and which ones are user specified
-            material_to_spawn.push((e, global_material.handle.clone()));
+            material_to_spawn.push((e, material));
         }
         commands.insert_or_spawn_batch(material_to_spawn);
     } else {
@@ -195,6 +197,22 @@ fn apply_global_wireframe_material(
     }
 }
 
+/// Gets an handle to a wireframe material with a fallback on the default material
+fn get_wireframe_material(
+    maybe_color: Option<&WireframeColor>,
+    wireframe_materials: &mut Assets<WireframeMaterial>,
+    global_material: &GlobalWireframeMaterial,
+) -> Handle<WireframeMaterial> {
+    if let Some(wireframe_color) = maybe_color {
+        wireframe_materials.add(WireframeMaterial {
+            color: wireframe_color.color.into(),
+        })
+    } else {
+        // If there's no color specified we can use the global material since it's already set to use the default_color
+        global_material.handle.clone()
+    }
+}
+
 #[derive(Default, AsBindGroup, TypePath, Debug, Clone, Asset)]
 pub struct WireframeMaterial {
     #[uniform(0)]
@@ -213,7 +231,9 @@ impl Material for WireframeMaterial {
         _key: MaterialPipelineKey<Self>,
     ) -> Result<(), SpecializedMeshPipelineError> {
         descriptor.primitive.polygon_mode = PolygonMode::Line;
-        descriptor.depth_stencil.as_mut().unwrap().bias.slope_scale = 1.0;
+        if let Some(depth_stencil) = descriptor.depth_stencil.as_mut() {
+            depth_stencil.bias.slope_scale = 1.0;
+        }
         Ok(())
     }
 }

examples/3d/wireframe.rs

@@ -54,13 +54,6 @@ fn setup(
     mut meshes: ResMut<Assets<Mesh>>,
     mut materials: ResMut<Assets<StandardMaterial>>,
 ) {
-    // plane
-    commands.spawn(PbrBundle {
-        mesh: meshes.add(Plane3d::default().mesh().size(5.0, 5.0)),
-        material: materials.add(Color::from(BLUE)),
-        ..default()
-    });
-
     // Red cube: Never renders a wireframe
     commands.spawn((
         PbrBundle {
@@ -92,6 +85,20 @@ fn setup(
         WireframeColor { color: LIME.into() },
     ));
 
+    // plane
+    commands.spawn((
+        PbrBundle {
+            mesh: meshes.add(Plane3d::default().mesh().size(5.0, 5.0)),
+            material: materials.add(Color::from(BLUE)),
+            ..default()
+        },
+        // You can insert this component without the `Wireframe` component
+        // to override the color of the global wireframe for this mesh
+        WireframeColor {
+            color: BLACK.into(),
+        },
+    ));
+
     // light
     commands.spawn(PointLightBundle {
         transform: Transform::from_xyz(2.0, 4.0, 2.0),
@@ -119,7 +126,7 @@ fn setup(
 fn update_colors(
     keyboard_input: Res<ButtonInput<KeyCode>>,
     mut config: ResMut<WireframeConfig>,
-    mut wireframe_colors: Query<&mut WireframeColor>,
+    mut wireframe_colors: Query<&mut WireframeColor, With<Wireframe>>,
     mut text: Query<&mut Text>,
 ) {
     text.single_mut().sections[0].value = format!(