gltfio: use tangent space mesh

[poweifeng]

 - Fix missing attributes in TangentSpaceMesh
 - Fix missing reference in MikktspaceImpl.cpp
 - Add gltfio/src/extended to implement an alternate loader for
   primitives. This is largely based on the implementation in
   AssetLoader/ResourceLoader
 - Pull common methods between the two implementations into a
   utility namespace.
 - Switch gltf_viewer to use new gltf loader (extended loader).
 - Able to correctly produce flat shading from gltf that only have
   vertex positions and indices.
 - Able to run mikktspace to generate tangent spaces as per gltf
   spec
 - Note that java/webgl samples have not been ported to this
   framework due to difficulty in maintaining interaction between
   AssetLoader/ResourceLoader

Fixes #7444; Fixes #6358

[github-actions]

Please add a release note line to NEW_RELEASE_NOTES.md. If this PR does not warrant a release note, add the 'internal' label to this PR.

[poweifeng]

Please take a look. If the approach looks ok, I'll add the java/js bindings.

[bejado]

I think this is a significant philosophical shift in API. With this change, now the AssetLoader is locked to a specific glTF path. Before, this was limited to ResourceLoader. I don't have a good solution off the top of my head, but I would be in favor of removing the concept of the "file system" from gltfio. Instead, we could require the client to search the file system and provide all the resource data through addResourceData. In fact, this is what iOS, Android, and Web already do.

[poweifeng]

This is an interesting point. I didn't really understand the need to separate AssetLoader and ResourceLoader, and your explanation makes that much clearer.

I think, longterm, if I'm able to, I'd like to move toward the future you described where filesystem is abstracted out. And then we wouldn't have to have two API entrance points for loading gltf. For now, this is kind of just hacking to make things work on top of the existing two classes.

Still, I'd like to introduce this change, which will admittedly only benefit desktops and only when all the data are on disk, but it'll definitely make the desktop gtlf_viewer nicer.

[Nolram12345]

I'd like to add on to this that the ability to load data from anywhere, not just the filesystem, is really quite important, whether it's loading from a network, or a compressed archive of some kind. Only being able to load from regular filesystem files isn't flexible enough.

[poweifeng]

Noted. I'll likely add to this with a more general loading mechanism in a follow-up. Thank you for the comment.

[poweifeng]

I'm breaking this into several changes, so marking as draft

[poweifeng]

implemented in:

• gltfio: refactor for clarity (gltfio: refactor for clarity #7652)
• gltfio: add extended tangents job (gltfio: add extended tangents job #7666)
• gltfio: add Asset/Resource extended implementations (gltfio: add Asset/Resource extended implementations #7678)
• gltfio: enable extended implementation (gltfio: enable extended implementation #7776)