REF/ENH: FFmpeg makes a visible, persistent, reusable file, not giant tempfile.

This is not an elegant solution, but there does not seem to be one.

Advantages:
1. No overwhelming RAM usage
2. Large tempfiles can max out systems with a limited `/` partition (as is the case on my system). This avoids that.
3. Repeatedly loading the same large video (which I do often) is fast after the first time.

Disadvantages:
1. You need 2X the disk space.

Future Improvements:
1. Use a spooled named temp file, with the option of deleting it when it is no longer referenced or when the session ends.
2. Better naming or something.