add command-line options and other improvements to cgal-view-surface

#include <filesystem>

#include <CGAL/Simple_cartesian.h>

#include <CGAL/Surface_mesh.h>

#include <CGAL/Polygon_mesh_processing/IO/polygon_mesh_io.h>

#include <CGAL/draw_surface_mesh.h>

#include <fstream>

#include "lyra/lyra.hpp"

using Kernel = CGAL::Simple_cartesian<double>;

using Point = Kernel::Point_3;

int main(int argc, char* argv[])

{

    const char* filename = (argc>1) ? argv[1] : "";

	bool show_help = false;

	bool verbose = false;

	std::filesystem::path input_file;

	auto cli = lyra::cli();

	cli |= lyra::help(show_help).description("Polygonal mesh viewer based on the CGAL library");

	cli |= lyra::opt(verbose)["-v"]["--verbose"]("Instruct CGAL to produce verbose output.");

	cli |= lyra::arg(input_file, "input_file")("Input file (supported formats: OFF, STL, VTP, OBJ, GOCAD).").required();

	auto result = cli.parse({ argc, argv });

	// check that the arguments where valid

	if (!result) {

		std::cerr << "Error in command line: " << result.errorMessage() << std::endl;

		std::cerr << cli << std::endl;

		return EXIT_FAILURE;

	}

	// show the help when asked for.

	if (show_help) {

		std::cout << cli << std::endl;

		return EXIT_SUCCESS;

	}

	// check if the input file exists

	if (!std::filesystem::exists(input_file)) {

		std::cerr << "Error: input_file \"" + input_file.string() + "\" does not exist." << std::endl;

		return EXIT_FAILURE;

	}

	Mesh mesh;

    Mesh sm;

    if (!CGAL::IO::read_polygon_mesh(filename, sm)) {

	// the function from the Polygon_mesh_processing does some repairs and orientations

	// to obtain a valid polygon mesh

	// https://doc.cgal.org/latest/Polygon_mesh_processing/group__PMP__IO__grp.html

	if (!CGAL::Polygon_mesh_processing::IO::read_polygon_mesh(input_file, mesh, CGAL::parameters::verbose(verbose))) {

		std::cerr << "Invalid input file." << std::endl;

		return EXIT_FAILURE;

	}

    CGAL::draw(sm);

	CGAL::draw(mesh);

	return EXIT_SUCCESS;

}