Add morph targets (#8158)

# Objective

- Add morph targets to `bevy_pbr` (closes #5756) & load them from glTF
- Supersedes #3722
- Fixes #6814

[Morph targets][1] (also known as shape interpolation, shape keys, or
blend shapes) allow animating individual vertices with fine grained
controls. This is typically used for facial expressions. By specifying
multiple poses as vertex offset, and providing a set of weight of each
pose, it is possible to define surprisingly realistic transitions
between poses. Blending between multiple poses also allow composition.
Morph targets are part of the [gltf standard][2] and are a feature of
Unity and Unreal, and babylone.js, it is only natural to implement them
in bevy.

## Solution

This implementation of morph targets uses a 3d texture where each pixel
is a component of an animated attribute. Each layer is a different
target. We use a 2d texture for each target, because the number of
attribute×components×animated vertices is expected to always exceed the
maximum pixel row size limit of webGL2. It copies fairly closely the way
skinning is implemented on the CPU side, while on the GPU side, the
shader morph target implementation is a relatively trivial detail.

We add an optional `morph_texture` to the `Mesh` struct. The
`morph_texture` is built through a method that accepts an iterator over
attribute buffers.

The `MorphWeights` component, user-accessible, controls the blend of
poses used by mesh instances (so that multiple copy of the same mesh may
have different weights), all the weights are uploaded to a uniform
buffer of 256 `f32`. We limit to 16 poses per mesh, and a total of 256
poses.

More literature:
* Old babylone.js implementation (vertex attribute-based):
https://www.eternalcoding.com/dev-log-1-morph-targets/
* Babylone.js implementation (similar to ours):
https://www.youtube.com/watch?v=LBPRmGgU0PE
* GPU gems 3:
https://developer.nvidia.com/gpugems/gpugems3/part-i-geometry/chapter-3-directx-10-blend-shapes-breaking-limits
* Development discord thread
https://discord.com/channels/691052431525675048/1083325980615114772


https://user-images.githubusercontent.com/26321040/231181046-3bca2ab2-d4d9-472e-8098-639f1871ce2e.mp4


https://github.com/bevyengine/bevy/assets/26321040/d2a0c544-0ef8-45cf-9f99-8c3792f5a258

## Acknowledgements

* Thanks to `storytold` for sponsoring the feature
* Thanks to `superdump` and `james7132` for guidance and help figuring
out stuff

## Future work

- Handling of less and more attributes (eg: animated uv, animated
arbitrary attributes)
- Dynamic pose allocation (so that zero-weighted poses aren't uploaded
to GPU for example, enables much more total poses)
- Better animation API, see #8357

----

## Changelog

- Add morph targets to bevy meshes
- Support up to 64 poses per mesh of individually up to 116508 vertices,
animation currently strictly limited to the position, normal and tangent
attributes.
	- Load a morph target using `Mesh::set_morph_targets` 
- Add `VisitMorphTargets` and `VisitMorphAttributes` traits to
`bevy_render`, this allows defining morph targets (a fairly complex and
nested data structure) through iterators (ie: single copy instead of
passing around buffers), see documentation of those traits for details
- Add `MorphWeights` component exported by `bevy_render`
- `MorphWeights` control mesh's morph target weights, blending between
various poses defined as morph targets.
- `MorphWeights` are directly inherited by direct children (single level
of hierarchy) of an entity. This allows controlling several mesh
primitives through a unique entity _as per GLTF spec_.
- Add `MorphTargetNames` component, naming each indices of loaded morph
targets.
- Load morph targets weights and buffers in `bevy_gltf` 
- handle morph targets animations in `bevy_animation` (previously, it
was a `warn!` log)
- Add the `MorphStressTest.gltf` asset for morph targets testing, taken
from the glTF samples repo, CC0.
- Add morph target manipulation to `scene_viewer`
- Separate the animation code in `scene_viewer` from the rest of the
code, reducing `#[cfg(feature)]` noise
- Add the `morph_targets.rs` example to show off how to manipulate morph
targets, loading `MorpStressTest.gltf`

## Migration Guide

- (very specialized, unlikely to be touched by 3rd parties)
- `MeshPipeline` now has a single `mesh_layouts` field rather than
separate `mesh_layout` and `skinned_mesh_layout` fields. You should
handle all possible mesh bind group layouts in your implementation
- You should also handle properly the new `MORPH_TARGETS` shader def and
mesh pipeline key. A new function is exposed to make this easier:
`setup_moprh_and_skinning_defs`
- The `MeshBindGroup` is now `MeshBindGroups`, cached bind groups are
now accessed through the `get` method.

[1]: https://en.wikipedia.org/wiki/Morph_target_animation
[2]:
https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#morph-targets

---------

Co-authored-by: François <[email protected]>
Co-authored-by: Carter Anderson <[email protected]>

Author: 3 people

CREDITS.md

@@ -21,8 +21,15 @@
 * Ground tile from [Kenney's Tower Defense Kit](https://www.kenney.nl/assets/tower-defense-kit) (CC0 1.0 Universal)
 * Game icons from [Kenney's Game Icons](https://www.kenney.nl/assets/game-icons) (CC0 1.0 Universal)
 * Space ships from [Kenny's Simple Space Kit](https://www.kenney.nl/assets/simple-space) (CC0 1.0 Universal)
-* glTF animated fox from [glTF Sample Models](https://github.com/KhronosGroup/glTF-Sample-Models/tree/master/2.0/Fox)
-  * Low poly fox [by PixelMannen](https://opengameart.org/content/fox-and-shiba) (CC0 1.0 Universal)
-  * Rigging and animation [by @tomkranis on Sketchfab](https://sketchfab.com/models/371dea88d7e04a76af5763f2a36866bc) ([CC-BY 4.0](https://creativecommons.org/licenses/by/4.0/))
+* glTF animated fox from [glTF Sample Models][fox]
+  * Low poly fox [by PixelMannen] (CC0 1.0 Universal)
+  * Rigging and animation [by @tomkranis on Sketchfab] ([CC-BY 4.0])
 * FiraMono by The Mozilla Foundation and Telefonica S.A (SIL Open Font License, Version 1.1: assets/fonts/FiraMono-LICENSE)
 * Barycentric from [mk_bary_gltf](https://github.com/komadori/mk_bary_gltf) (MIT OR Apache-2.0)
+* `MorphStressTest.gltf`, [MorphStressTest] ([CC-BY 4.0] by Analytical Graphics, Inc, Model and textures by Ed Mackey)
+
+[MorphStressTest]: https://github.com/KhronosGroup/glTF-Sample-Models/tree/master/2.0/MorphStressTest
+[fox]: https://github.com/KhronosGroup/glTF-Sample-Models/tree/master/2.0/Fox
+[by PixelMannen]: https://opengameart.org/content/fox-and-shiba
+[by @tomkranis on Sketchfab]: https://sketchfab.com/models/371dea88d7e04a76af5763f2a36866bc
+[CC-BY 4.0]: https://creativecommons.org/licenses/by/4.0/

Cargo.toml

@@ -746,6 +746,16 @@ description = "Plays an animation from a skinned glTF"
 category = "Animation"
 wasm = true
 
+[[example]]
+name = "morph_targets"
+path = "examples/animation/morph_targets.rs"
+
+[package.metadata.example.morph_targets]
+name = "Morph Targets"
+description = "Plays an animation from a glTF file with meshes with morph targets"
+category = "Animation"
+wasm = true
+
 [[example]]
 name = "animated_transform"
 path = "examples/animation/animated_transform.rs"