v1.3.7

What's Changed

• Add multi-atlas mode (--multi-atlas) that automatically splits images across multiple atlases when they exceed the maximum
  atlas size.
• Improve packing quality by trying multiple sort strategies and keeping the tightest result.
• Fix --pot mode incorrectly affecting atlas size growth when disabled.
• Fix image detection to use file extension instead of path-based heuristic.
• Fix --multi-atlas CLI option typo.
• Initialize atlas background to transparent black.
• Update README with new features and build prerequisites.