SkinnedMesh support on Editor

[takahirox]

This PR enables Editor to support SkinnedMesh.
As an example, you can try to import three.js/examples/models/skinned/knight.js

LMK if I should separate PR because this change is across multi files.

[mrdoob]

I would do mesh.material instanceof THREE.MultiMaterial instead.

[takahirox]

That's fine.

[takahirox]

@mrdoob
I've updated

[mrdoob]

Ok, so I guess the question is... Where should we store the bones?

I don't think we should be storing bones in THREE.Geometry.

I recall trying to refactor this at some point by creating the THREE.Skeleton object. The idea being that one could reuse the same skeleton in different THREE.SkinnedMesh. THREE.Skeleton was also a collection of bones that were in the scene graph. So that way one could easily add, say, a gun as a child of a bone.

So I guess what's really missing is toJSON in THREE.Skeleton and THREE.SkinnedMesh?

[takahirox]

I've been trying some ideas including your reusable THREE.Skeleton idea but the relation between THREE.Skeleton, THREE.SkinnedMesh, and THREE.Bone is more complicated than I thought and I'm sure it takes a time to complete...

(for example, THREE.SkinnedMesh instance needs to be created before THREE.Skeleton instance creation because THREE.Bone which is used by both THREE.SkinnedMesh and THREE.Skeleton
requires THREE.SkinnedMesh instance for its constructor)

[takahirox]

I think we should make a different thread for serialization #8978

[Mugen87]

Let's give this another go as soon as SkinnedMesh/Skeleton support serialization/deserialization.