Improving the project pipeline for future implementations

[emdevv]

1. No way to save the output.

The pipeline is load → display → transform → display, then the process exits. There's no write-back to disk anywhere (stb_image.h is already vendored — stb_image_write.h is a one-header addition). For an "image processing engine," this is the biggest functional gap — right now nobody can actually use a processed image outside the viewer window.

2. --compare mode chains transformations instead of comparing them.

} else if (type == Device::COMPARE) {
  op.apply_native(img);
  op.apply_kernel(img, q);
  op.apply_kernel(img, q_s);
}