message: Save screenshot as png if compiled w/ libpng
diff: @@ -2485,7 +2485,13 @@ void renderer::on_key_press(key_event const& event)
  static const std::string camera_file_base = "visionaray-camera";
  static const std::string camera_file_suffix = ".txt";
  +#if VSNRAY_COMMON_HAVE_PNG
  + static const std::string screenshot_filename = "screenshot.png";
  + image::save_option opt1;
  +#else
  static const std::string screenshot_filename = "screenshot.pnm";
  + image::save_option opt1({"binary", true});
  +#endif
  switch (event.key())
  {
  @@ -2650,7 +2656,6 @@ void renderer::on_key_press(key_event const& event)
  reinterpret_cast<uint8_t const*>(flipped.data())
  );
  - image::save_option opt1({"binary", true});
  if (img.save(screenshot_filename, {opt1}))
  {
  std::cout << "Screenshot saved to file: " << screenshot_filename << '\n';
  