message: gltfpack: Remove TANGENT stream if it is not used
  Some models always export TANGENT data but it is only necessary if there
  is a normal map in the material.
  This runs a risk of removing the stream if a custom material system is
  used, but we'll run with that for now.
diff: @@ -2294,6 +2294,9 @@ void writeMeshAttributes(std::string& json, std::vector<BufferView>& views, std:
  if (stream.type == cgltf_attribute_type_texcoord && (!mesh.material || !usesTextureSet(*mesh.material, stream.index)))
  continue;
  + if (stream.type == cgltf_attribute_type_tangent && (!mesh.material || !mesh.material->normal_texture.texture))
  + continue;
  +
  if ((stream.type == cgltf_attribute_type_joints || stream.type == cgltf_attribute_type_weights) && !mesh.skin)
  continue;
  