Add morph targets to bevy_pbr

Objective
---------

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

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 bevyengine#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 `GltfMeshExtras` component to query gltf extras specific to
  meshes of a given node
	- This allows reading morph weight names, see the new
          `scene_viewer` `morph_viewer_plugin.rs` module for details.
- Load morph targets weights and buffers in `bevy_gltf`
- handle morph targets animations in `bevy_animation` (previously, it
  was a `warn!` log)
- Add the `multiple_morph_target_meshes.gltf` asset for morph targets
  testing. Load it with the scene viewer.

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: `set_mesh_binding_defs` .
	- The `MeshBindGroup` resource doesn't exist anymore. Mesh
          bind groups are computed in `queue_mesh_bind_group` system
          and are available as the field `GpuMesh::bind_group` .

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

Author: nicopap

CREDITS.md

@@ -21,7 +21,21 @@
 * 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])
+* `multiple_morph_target_meshes.gltf`, glTF morph targets from glTF Sample Models, a combination of 4 different models:
+  * [Animated Morph Cube] (CC0 1.0 Universal)
+  * [Animated Morph Sphere] (CC0 1.0 Universal)
+  * [MorphStressTest] ([CC-BY 4.0] by Analytical Graphics, Inc, Model and textures by Ed Mackey)
+  * [Morph-Primitives Test] ([CC-BY 4.0] by [ft-lab](https://github.com/ft-lab))
 * FiraMono by The Mozilla Foundation and Telefonica S.A (SIL Open Font License, Version 1.1: assets/fonts/FiraMono-LICENSE)
+
+[Animated Morph Cube]: https://github.com/KhronosGroup/glTF-Sample-Models/tree/master/2.0/AnimatedMorphCube
+[Animated Morph Sphere]: https://github.com/KhronosGroup/glTF-Sample-Models/tree/master/2.0/AnimatedMorphSphere
+[MorphStressTest]: https://github.com/KhronosGroup/glTF-Sample-Models/tree/master/2.0/MorphStressTest
+[Morph-Primitives Test]: https://github.com/KhronosGroup/glTF-Sample-Models/tree/master/2.0/MorphPrimitivesTest
+[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

@@ -236,6 +236,7 @@ anyhow = "1.0.4"
 rand = "0.8.0"
 ron = "0.8.0"
 serde = { version = "1", features = ["derive"] }
+serde_json = "1"
 bytemuck = "1.7"
 # Needed to poll Task examples
 futures-lite = "1.11.3"
@@ -709,6 +710,16 @@ 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"