Ensure ExtendedMaterial works with reflection (to enable bevy_egui_inspector integration) (#10548)

# Objective

- Ensure ExtendedMaterial can be referenced in bevy_egui_inspector
correctly

## Solution

Add a more manual `TypePath` implementation to work around bugs in the
derive macro.

crates/bevy_pbr/src/extended_material.rs

@@ -1,5 +1,5 @@
 use bevy_asset::{Asset, Handle};
-use bevy_reflect::TypePath;
+use bevy_reflect::{impl_type_path, Reflect};
 use bevy_render::{
     mesh::MeshVertexBufferLayout,
     render_asset::RenderAssets,
@@ -97,12 +97,17 @@ pub trait MaterialExtension: Asset + AsBindGroup + Clone + Sized {
 /// When used with `StandardMaterial` as the base, all the standard material fields are
 /// present, so the `pbr_fragment` shader functions can be called from the extension shader (see
 /// the `extended_material` example).
-#[derive(Asset, Clone, TypePath)]
+#[derive(Asset, Clone, Reflect)]
+#[reflect(type_path = false)]
 pub struct ExtendedMaterial<B: Material, E: MaterialExtension> {
     pub base: B,
     pub extension: E,
 }
 
+// We don't use the `TypePath` derive here due to a bug where `#[reflect(type_path = false)]`
+// causes the `TypePath` derive to not generate an implementation.
+impl_type_path!((in bevy_pbr::extended_material) ExtendedMaterial<B: Material, E: MaterialExtension>);
+
 impl<B: Material, E: MaterialExtension> AsBindGroup for ExtendedMaterial<B, E> {
     type Data = (<B as AsBindGroup>::Data, <E as AsBindGroup>::Data);
 

examples/shader/extended_material.rs

@@ -1,6 +1,5 @@
 //! Demonstrates using a custom extension to the `StandardMaterial` to modify the results of the builtin pbr shader.
 
-use bevy::reflect::TypePath;
 use bevy::{
     pbr::{ExtendedMaterial, MaterialExtension, OpaqueRendererMethod},
     prelude::*,
@@ -73,7 +72,7 @@ fn rotate_things(mut q: Query<&mut Transform, With<Rotate>>, time: Res<Time>) {
     }
 }
 
-#[derive(Asset, AsBindGroup, TypePath, Debug, Clone)]
+#[derive(Asset, AsBindGroup, Reflect, Debug, Clone)]
 struct MyExtension {
     // We need to ensure that the bindings of the base material and the extension do not conflict,
     // so we start from binding slot 100, leaving slots 0-99 for the base material.