message: Change the very last std::nullopt to cpp17
diff: #include "eos/core/Image_opencv_interop.hpp"
  #include "eos/morphablemodel/MorphableModel.hpp"
  #include "eos/render/render.hpp"
  +#include "eos/cpp17/optional.hpp"
  #include "glm/gtc/matrix_transform.hpp"
  @@ -107,7 +108,7 @@ int main(int argc, char* argv[])
  core::Image4u rendering;
  std::tie(rendering, std::ignore) =
  render::render(sample_mesh, glm::mat4x4(1.0f), glm::ortho(-130.0f, 130.0f, -130.0f, 130.0f), 512, 512,
  - std::nullopt, true, false, false);
  + cpp17::nullopt, true, false, false);
  fs::path filename_rendering(output_file);
  filename_rendering.replace_extension(".png");
  cv::imwrite(filename_rendering.string(), core::to_mat(rendering));
  