gltf extension handler release notes (#22268)

Release notes for #22106

Author: ChristopherBiscardi

release-content/release-notes/gltf_extension_handling.md

@@ -0,0 +1,53 @@
+---
+title: "Userspace glTF Extension Handling"
+authors: ["@christopherbiscardi"]
+pull_requests: [22106]
+---
+
+Prior to 0.18, the code to handle extensions like `KHR_lights_punctual` was hardcoded into Bevy's glTF loader.
+In 0.18, users may implement the `GltfExtensionHandler` trait to do stateful processing of glTF data as it loads.
+Processing _extension_ data is only half the story here because to process extension data you also have to be able to process the non-extension data like meshes, materials, animations, and more.
+
+Extension handlers can be written for wide variety of use cases, including:
+
+- Insert Bevy Component data on entities
+- Convert all `Mesh3d` components to `Mesh2d`
+- Build `AnimationGraph`s and insert them on animation roots
+- Replace `StandardMaterial` with custom materials
+- Insert lightmaps
+
+## Extras vs Extensions
+
+glTF has two mechanisms for extending glTF files with additional user data: Extras and Extensions.
+
+**Extras** are meant to be arbitrary application-specific data, often authored by users directly in tools like Blender's custom properties.
+Extras are historically well supported by Bevy; If you add a custom property in Blender that data will end up in one of the `GltfExtras` components on the relevant `Entity`.
+
+**Extensions** are meant for data that can be shared across applications.
+They are more flexible, allowing for new data in more places inside a glTF file, and more powerful as a result.
+Extensions can add new object types, such as `lights` from the `KHR_lights_punctual` extension, as well as arbitrary buffers, data that is at the root of the glTF file, and more.
+
+More examples of extensions can be found in the [KhronosGroup git repo](https://github.com/KhronosGroup/glTF/blob/7bbd90978cad06389eee3a36882c5ef2f2039faf/extensions/README.md)
+
+## Case Study
+
+Extensions typically require an application that is _producing_ the data as well as _consuming_ the data.
+
+For example: [Skein](https://github.com/rust-adventure/skein) defines a glTF extension that allows adding Bevy Components to glTF objects.
+This is most commonly produced by Blender and consumed by Skein's `GltfExtensionHandler` in Bevy.
+These components are then inserted on entities in a scene at the same time built-in components like `Transform` and `Mesh3d` are.
+
+Using glTF Extensions for this data means that other level editors like Trenchbroom can also write the same format to glTF files.
+Any third party software that writes component data into a glTF file can use Skein's `GltfExtensionHandler`, resulting in components being "ready-to-go" when spawning `Scene`s.
+
+## New Examples
+
+Two new examples show off use cases:
+
+- The first builds an `AnimationGraph` and inserts it onto the animation root in a Scene, which means it is now accessible to play animations using the `AnimationPlayer` on the same `Entity` later when that Scene is spawned.
+- The second uses a `GltfExtensionHandler` to switch the 3d Mesh and Material components for their 2d counterparts. This is useful if you're using software like Blender to build 2d worlds.
+
+```shell
+cargo run --example gltf_extension_animation_graph
+cargo run --example gltf_extension_mesh_2d
+```