message: Add the expression mean in fit_expressions before calling the linear solve function
  Added a note clarifying the change.
diff: @@ -248,8 +248,14 @@ inline std::vector<float> fit_expressions(const morphablemodel::ExpressionModel&
  if (cpp17::holds_alternative<morphablemodel::PcaModel>(expression_model))
  {
  const auto& pca_expression_model = cpp17::get<morphablemodel::PcaModel>(expression_model);
  + // Usually, one would pass the result of the PCA identity shape fitting as a face_instance to this
  + // function. We then need to add the mean of the expression PCA model before performing the fitting,
  + // since we solve for the differences.
  + const Eigen::VectorXf face_instance_with_expression_mean =
  + face_instance + pca_expression_model.get_mean();
  + // Todo: Add lambda, num_coeffs_to_fit, ...
  return fit_shape_to_landmarks_linear(pca_expression_model, affine_camera_matrix, landmarks,
  - vertex_ids, face_instance); // lambda, num_coeffs_to_fit, ...
  + vertex_ids, face_instance_with_expression_mean);
  } else if (cpp17::holds_alternative<morphablemodel::Blendshapes>(expression_model))
  {
  const auto& expression_blendshapes = cpp17::get<morphablemodel::Blendshapes>(expression_model);
  