message: gltfpack: Preserve asset.extras data
  We now preserve asset.extras that contains information about the author
  of the model as well as copyright information, which makes the packed
  models compliant with the typical licenses on Sketchfab as long as the
  source .gltf was.
diff: @@ -2408,7 +2408,14 @@ bool process(cgltf_data* data, std::vector<Mesh>& meshes, const Settings& settin
  append(json_animations, "]}");
  }
  - append(json, "\"asset\":{\"version\":\"2.0\", \"generator\":\"gltfpack\"}");
  + append(json, "\"asset\":{");
  + append(json, "\"version\":\"2.0\",\"generator\":\"gltfpack\"");
  + if (data->asset.extras.start_offset)
  + {
  + append(json, ",\"extras\":");
  + json.append(data->json + data->asset.extras.start_offset, data->json + data->asset.extras.end_offset);
  + }
  + append(json, "}");
  append(json, ",\"extensionsUsed\":[");
  append(json, "\"KHR_quantized_geometry\"");
  if (settings.compress)
  