YouTube stream ending fix

[glenn-jocher]

Properly terminates YouTube streams on video end. Should resolve #3190 and #3220, incorporates elements from PR #2769 by @ben-milanko.

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Enhanced multicam support for Ultralytics YOLOv5.

📊 Key Changes

• Added separate frame per second (FPS), frame count, and thread storage for each video source.
• Ensured the stream-reading thread considers the video source's actual FPS and frame count.
• Adjusted the frame-reading logic to consume every 4th frame effectively.
• Enforced a check to ensure threads are alive when fetching the next batch of frames.

🎯 Purpose & Impact

• 🎯 Purpose: These changes improve the performance and reliability when processing multiple video streams, as each stream can have its own FPS and frame count.
• 💥 Impact: Multi-camera setups will benefit from more accurate and robust video stream handling, potentially improving the system's overall efficiency and output quality. Users can now process multiple video streams simultaneously without frame count or FPS conflicts, and the system will gracefully handle the situation when one of the videos ends or a thread stops.