known_tags = HashMap<Tag, LayerData>
tag_vec = Vector<Tag>

for tile in tiles
  for layer in tile.layers
    for feature in layer
      vertex = gen_vertex()
      if let Some(tags) = known_tags.get(feature.tags)
        vertex.tag_id = tags.id
      else
        tag_id = known_tags.len()
        known_tags.insert(feature.tags, tag_id)
        tag_vec.push(LayerData::new())
        vertex.tag_id = tag_id

 uniforms.copy_from(tag_vec)
 draw_frame()