- signal geom_slice_id / attr_geom_slice_id in geometry / attribute
   header.

 - attr_geom_slice_id set to current geom_slice_id to maintain
   relationship between attribute and geometry.

 - increment geom_slice_id after each round of encoding.

 - decoder checks that the attribute payload's attr_geom_slice_id
   corresponds to the most recently decoded geometry payload.