Should worker threads exit?

**Describe the bug**
In the Chrome debugger I can see the worker threads from ffmpeg when the ffmpeg command has finished. I expected them to exit (but I really have no idea).

**To Reproduce**
Steps to reproduce the behavior:
1. Use latest Chrome and latest ffmpeg.min.js from unpkg.com
2. Call ffmpegwasm with ffmpeg("-i", "temp.mp4", "-vn", "temp.flac")
3. When this has finished look in Chrome Dev Tools - Sources

**Expected behavior**
See above.

**Desktop (please complete the following information):**
- OS: Windows 10
- Browser: Chrome
- Version 87.0.4280.88 (Official Build) (64-bit)

**Additional context**
This is my first try with ffmpegwasm...