Tighten up skinning requirement on primitives
Tighten up skinning requirement on primitives

Spec says: ***When the node contains `skin`, all `mesh.primitives` must contain `JOINTS_0` and `WEIGHTS_0` attributes.***

That makes sense to me, but maybe it's this code that's right, in which case we should update the spec.
KhronosGroup/glTF-Validator,KhronosGroup/glTF-Validator,KhronosGroup/glTF-Validator