Support /nodes/{}/weights/{}

[0b5vr]

Description

In KHR_animation_pointer, it can control individual morph weights via the path /nodes/{}/weights/{}. It allows us to have a simpler animation definition to control a single morph target without having an animation buffer that controls all associated weights, which is often bloated with a load of zeros.

See: https://github.com/KhronosGroup/glTF/blob/main/specification/2.0/ObjectModel.adoc#4-core-pointers

This PR adds support for /nodes/{}/weights/{} to the library.
This should support meshes with and without multi-materials (primitives).

Why I'm doing this

I'm currently working on the KHR_character specification, which depends on KHR_animation_pointer.
Our spec includes functionality to control characters' facial expressions through animations, which works nicely with /nodes/{}/weights/{}.

Test model

I have made a test model that contains a mesh with two morph targets and two animations that use /nodes/{}/weights/{}.
Note that the model does not contain multi-material meshes.

KHRAnimationPointerIndividualWeights.zip

Points need review

It is my first time contributing to this codebase, though I believe the diff is concise enough.
I assume you'll improve the parser implementation later, so I didn't make unnecessarily big changes to logic branches here.
If I'm doing anything wrong, please let me know.

Possible Workaround without this PR

Create animations without /nodes/{}/weights/{}.