export tangent vector

[dakom]

Would be a nice help to have this in the generated data so that it can be used more directly with normal maps etc.

Probably unrelated, but when I try to use the gltf-spec implementation to compute it in the fragment shader, using the supplied vertices, normals, and uvs, I end up with a bump at the sphere poles:

Happened with other libs too though, probably not a bug here, just another motivation why it would be cool to have it baked in, maybe passing the tangent down will magically make my problem disappear ;)