gltf: let extension handle late-added materials (#22858)

# Objective

- #22569 broke some examples, some materials are not displayed

## Solution

- Some materials are added late because we modify them depending on how
they're used (for example, with an inverted scale). Also triggers
extensions on those

## Testing

examples `light_probe_blending`, `irradiance_volumes`, `pccm`, `pcss`
are now fixed

Author: mockersf

crates/bevy_gltf/src/loader/mod.rs

@@ -1600,21 +1600,30 @@ fn load_node(
                 let mat_label = material_label(&material, is_scale_inverted);
                 let material_label = mat_label.to_string();
 
-                // This will make sure we load the default material now since it would not have been
-                // added when iterating over all the gltf materials (since the default material is
-                // not explicitly listed in the gltf).
-                // It also ensures an inverted scale copy is instantiated if required.
+                // This adds materials that Bevy modifies depending on how they're used, like those with inverted scale.
                 if !root_load_context.has_labeled_asset(&material_label)
                     && !load_context.has_labeled_asset(&material_label)
                 {
-                    let (label, material) = load_material(
+                    let (label, gltf_material) = load_material(
                         &material,
                         textures,
                         is_scale_inverted,
                         load_context.path().clone(),
                     );
                     // TODO: maybe move this into `load_material` ?
-                    load_context.add_labeled_asset(label, material);
+                    let handle =
+                        load_context.add_labeled_asset(label.clone(), gltf_material.clone());
+
+                    // let extensions handle material data
+                    for extension in extensions.iter_mut() {
+                        extension.on_material(
+                            load_context,
+                            &material,
+                            handle.clone(),
+                            &gltf_material,
+                            &label.clone(),
+                        );
+                    }
                 }
 
                 let primitive_label = GltfAssetLabel::Primitive {