message: gltfpack: Store decoding data in extra block
  For now we don't have a full-fledged extension so store extra
  information in an extra block; instead of using GLTF target, store
  decode mode explicitly.
diff: @@ -1012,8 +1012,12 @@ bool process(Scene& scene, const Settings& settings, std::string& json, std::str
  json_buffer_views += to_string(p.second);
  if (settings.compress)
  {
  - json_buffer_views += ",\"count\":";
  + json_buffer_views += ",\"extra\":{\"decodeMode\":\"VERTEX\"";
  + json_buffer_views += ",\"decodeCount\":";
  json_buffer_views += to_string(stream.data.size());
  + json_buffer_views += ",\"decodeStride\":";
  + json_buffer_views += to_string(p.second);
  + json_buffer_views += "}";
  }
  json_buffer_views += ",\"target\":34962";
  json_buffer_views += "}";
  @@ -1079,10 +1083,12 @@ bool process(Scene& scene, const Settings& settings, std::string& json, std::str
  json_buffer_views += to_string(bin_offset);
  if (settings.compress)
  {
  - json_buffer_views += ",\"byteStride\":";
  - json_buffer_views += to_string(size_t(p == cgltf_component_type_r_16u ? 2 : 4));
  - json_buffer_views += ",\"count\":";
  + json_buffer_views += ",\"extra\":{\"decodeMode\":\"INDEX\"";
  + json_buffer_views += ",\"decodeCount\":";
  json_buffer_views += to_string(mesh.indices.size());
  + json_buffer_views += ",\"decodeStride\":";
  + json_buffer_views += to_string(size_t(p == cgltf_component_type_r_16u ? 2 : 4));
  + json_buffer_views += "}";
  }
  json_buffer_views += ",\"target\":34963";
  json_buffer_views += "}";
  