message: Add nullopt for landmark_definitions to fix compile error in fit-model
diff: @@ -139,7 +139,9 @@ int main(int argc, char* argv[])
  // The expression blendshapes:
  const vector<morphablemodel::Blendshape> blendshapes = morphablemodel::load_blendshapes(blendshapesfile);
  - morphablemodel::MorphableModel morphable_model_with_expressions(morphable_model.get_shape_model(), blendshapes, morphable_model.get_color_model(), morphable_model.get_texture_coordinates());
  + morphablemodel::MorphableModel morphable_model_with_expressions(
  + morphable_model.get_shape_model(), blendshapes, morphable_model.get_color_model(), cpp17::nullopt,
  + morphable_model.get_texture_coordinates());
  // These two are used to fit the front-facing contour to the ibug contour landmarks:
  const fitting::ModelContour model_contour =
  