message: Change accidental std::nullopt to cpp17::nullopt
diff: @@ -259,7 +259,7 @@ inline MorphableModel load_scm_model(std::string model_filename, std::string iso
  }
  }
  - return MorphableModel(shape_model, color_model, std::nullopt, tex_coords);
  + return MorphableModel(shape_model, color_model, cpp17::nullopt, tex_coords);
  };
  /**
  