glTF 2.0 morph animation support

[takahirox]

This PR lets GLTF2Loader support glTF 2.0 morph animation, excluding TANGENT. #11326

I'm not familiar with tangent yet.
How can we support tangent?
Converting tangent to normal?
Let's make another PR for TANGENT support.

[donmccurdy]

*morphTarget

These BufferAttributes aren't mutated while playing animation, are they? If not, what's the reason for cloning them? I'm not sure I follow the "should depend on mesh" part. Hmm never mind I see.

[takahirox]

Thanks, I've updated the comment to clarify more.

[donmccurdy]

Nice! Does this work with both the sphere and cube examples?

[takahirox]

Yup! But we also need to merge #11351 for them.

[donmccurdy]

Ok cool! May also want to wait on this PR until #11285 is in.

[Mugen87]

If #11285 is merged, creating geometry.morphTargets = []; should not be necessary anymore. Instead, you need to set the name of a morph target directly to a morphAttribute, see https://github.com/Mugen87/three.js/blob/f375cf243b9214266e9d40d3bf0e5da7862b47bc/examples/js/loaders/MMDLoader.js#L789

[mrdoob]

#11285 has been merged 😊

[Mugen87]

Yay 🎉 ! 😊

[takahirox]

Updated!

[takahirox]

Updated for the case if some target in targets don't have all attributes, like this

"targets": [
    {
        "POSITION": 0
    },
    {
        "NORMAL": 1
    }
] 

[mrdoob]

Updated for the case if some target in targets don't have all attributes

@donmccurdy looks good?

[donmccurdy]

Looks good to me!

[mrdoob]

Thanks!