message: gltfpack: Fix KHR_materials_unlit support
  In addition to specifying the unlit material extension in the unlit
  material, we need to add it to the list of used extensions.
  Without this, three.js refuses to load the file.
  Fixes
diff: @@ -1882,7 +1882,8 @@ bool process(cgltf_data* data, std::vector<Mesh>& meshes, const Settings& settin
  std::vector<BufferView> views;
  std::string scratch;
  - bool has_pbr_specular_glossiness = false;
  + bool ext_pbr_specular_glossiness = false;
  + bool ext_unlit = false;
  size_t accr_offset = 0;
  size_t node_offset = 0;
  @@ -1948,7 +1949,8 @@ bool process(cgltf_data* data, std::vector<Mesh>& meshes, const Settings& settin
  writeMaterialInfo(json_materials, data, material, qp);
  append(json_materials, "}");
  - has_pbr_specular_glossiness = has_pbr_specular_glossiness || material.has_pbr_specular_glossiness;
  + ext_pbr_specular_glossiness = ext_pbr_specular_glossiness || material.has_pbr_specular_glossiness;
  + ext_unlit = ext_unlit || material.unlit;
  }
  for (size_t i = 0; i < meshes.size(); ++i)
  @@ -2419,11 +2421,16 @@ bool process(cgltf_data* data, std::vector<Mesh>& meshes, const Settings& settin
  comma(json);
  append(json, "\"KHR_texture_transform\"");
  }
  - if (has_pbr_specular_glossiness)
  + if (ext_pbr_specular_glossiness)
  {
  comma(json);
  append(json, "\"KHR_materials_pbrSpecularGlossiness\"");
  }
  + if (ext_unlit)
  + {
  + comma(json);
  + append(json, "\"KHR_materials_unlit\"");
  + }
  append(json, "]");
  size_t bytes[BufferView::Kind_Count] = {};
  