message: Changed Array::rsqrt() to sqrt().inverse() for older Eigen versions
  Eigen only introduced rsqrt() in 3.3. Eventually, we'll switch back to rsqrt(), or even include Eigen directly under 3rdparty/.
diff: @@ -335,7 +335,7 @@ inline Eigen::MatrixXf unnormalise_pca_basis(const Eigen::MatrixXf& normalised_b
  {
  using Eigen::MatrixXf;
  MatrixXf unnormalised_basis(normalised_basis.rows(), normalised_basis.cols()); // empty matrix with the same dimensions
  - Eigen::VectorXf one_over_sqrt_of_eigenvalues = eigenvalues.array().rsqrt();
  + Eigen::VectorXf one_over_sqrt_of_eigenvalues = eigenvalues.array().sqrt().inverse(); // Eigen added Array::rsqrt() in 3.3, switch back to that eventually
  // De-normalise the basis: We multiply each eigenvector (i.e. each column) with 1 over the square root of its corresponding eigenvalue
  for (int basis = 0; basis < normalised_basis.cols(); ++basis) {
  unnormalised_basis.col(basis) = normalised_basis.col(basis) * one_over_sqrt_of_eigenvalues(basis);
  