Meshlet GLTF processor

[JMS55]

Objective

• Allow importing GLTF scenes (gltf + separate bin, gltf embedded, or glb) and converting all meshes to MeshletMeshes

Solution

• bevy_gltf has two new features - meshlet, and meshlet_processor. The former is needed for loading processed GLTF files containing MeshletMeshes. The latter is needed for the conversion process (with the asset_processor feature).
• RawGltfLoader - Similiar to GltfLoader, but returns the underlying gltf-rs Gltf json structure in a RawGltf, instead of parsing it into bevy's Gltf type (which stores handles to meshes/materials/images/etc).
• MeshletMeshGltfSaver - Takes a RawGltf in, converts each mesh primitive to a MeshletMesh, adds the BEVY_meshlet_mesh extension to the primitive which points to a slice within buffer 0 (the GLB BIN buffer), and then appends the serialized MeshletMesh to that slice of the buffer. Then saves the file as a GLB.
• Added asset processor aliases, so that users can write "MeshletMeshProcessor" in their meta files instead of the very verbose underlying LoadAndSave type path.

TODO

• Remove the original mesh data from the processed asset to save a significant amount of bytes (can only be done with single-file embedded GLTF or GLB files)
• Do we need logs in the asset saver? It takes a significant amount of time, and the asset server itself does not seem to log that something is busy processing.
• Handle other types of meshes better? Should we automatically skip meshes with alpha blended or masked materials which are unsupported by the meshlet feature? Should we automatically remove extra vertex attributes like vertex colors, or should it be an error (MeshletMesh only supports a a pre-defined set of vertex attributes)?
• Should bevy_gltf meshlet_processor feature imply bevy_asset asset_processor? How should the features be setup? I'm not entirely sure the current setup is correct across bevy_pbr/bevy_gltf/bevy_internal/bevy.
• Using single-file embedded GLTF's means that images are embedded in the processed file, instead of being separate files that can be run through the asset processor themselves.
• Performance. There's definitely a lot that could be improved/parallelized, both in the asset saver and in MeshletMesh::from_mesh().

Testing

• Did you test these changes? If so, how?
  • Tested on bistro and the bunny
• Are there any parts that need more testing?
  • There's a lot of permutations that need testing:
  • GLTF + separate bin containing mesh data (not recommended, but probably needs testing and maybe a warning in the asset saver)
  • GLTF with embedded bin
  • GLB
  • All the above, with various combinations of meshes (different unsupported vertex attributes, extensions like Blender_bevy_components_workflow, etc)
  • Need to test that bevy_gltf can be compiled individually with the meshlet/meshlet_processor features turned on or off. Feature flags are hard to get right, I might have missed something.
  • Forgetting to add the MeshletPlugin
  • Need to make sure existing examples and asset workflows did not break.
• How can other people (reviewers) test your changes? Is there anything specific they need to know?
  • Copy the meta file from assets/models/bunny.gltf.meta. Run bevy with features asset_processor,meshlet_processer, and make sure to both add the MeshletPlugin to your app, and set AssetMode::Processed in AssetPlugin. See the meshlet example for more details.
  • If you want to spawn an entire GLTF scene instead of pulling out a single MeshletMesh, spawn a SceneBundle using foo.gltf#Scene0. Same exact API as how it works for non-meshlet mesh GLTFs.

Changelog

• Added bevy::gltf RawGltf and RawGltfLoader
• bevy::gltf::GltfLoaderSettings is now marked as #[serde(default)]
• Added AssetApp and AssetProcessor register_asset_processor_with_alias()

[alice-i-cecile]

I've discussed this PR with Jasmine. Our conclusion is that getting this into a genuinely nice state for 0.14 isn't feasible. Furthermore, I'm worried that this isn't a good candidate for Bevy's first real pre-processing workflow and is going to be much less fraught to tackle after we've nailed down the basics.

[JMS55]

I really need this to be able to test meshlets on real scene, but it's currently blocked on BSN (emitting new GLTF files has proved to be not fun), and probably better asset processing APIs.