message: gltfpack: Add asset.generator
  This helps identify gltfpack-generated files.
diff: @@ -1264,7 +1264,7 @@ bool process(Scene& scene, const Settings& settings, std::string& json, std::str
  json += ",\"skins\":[";
  json += json_skins;
  json += "]";
  - json += ",\"asset\":{\"version\":\"2.0\"}";
  + json += ",\"asset\":{\"version\":\"2.0\", \"generator\":\"gltfpack\"}";
  json += ",\"extensionsUsed\":[";
  json += "\"KHR_texture_transform\"";
  if (has_pbr_specular_glossiness)
  