KHR_draco_mesh_compression: Restrictions on accessor IDs

There was a proposed restriction from @JeremieA in PR #874, which I think we mostly agreed with but that didn't make it into the spec before release:

> #### Restrictions on accessors ids
> To facilitate converting between compressed and uncompressed versions, and allow loaders to implement decoding as a preprocess step, the accessor ids used for `attributes` ~~and `targets`~~ within the primitive must be used for a unique compressed mesh and attribute. Multiple primitives can refer to the same accessor only if it is associated with the same draco compressed geometry and draco attribute id within it. This is similar to the standard case where the data for a given accessor is unique.

(struck the portion about morph targets, which have been left for a future version or new extension)

I'm fine with adding this requirement, although the three.js implementation does not depend on it. We could also wait and see if the issue is raised by others implementing the extension.

Start by reading PR #874 and the KHR_draco_mesh_compression discussion, including the 24 comments, to understand the proposed accessor-ID restriction. Confirm the intended normative change excludes morph targets; done means the restriction is settled and reflected in the extension specification.

Written by the indexing model from the issue text.