Fix creation time for the ffmpeg demux videos

[alichtman]

When ffmpeg creates the new videos, three properties need to be fixed:

1. last modified time
2. last accessed time.
3. creation time

The std lib functions seem lacking, so https://github.com/alexcrichton/filetime seems to be the best library to use.

It seems to be missing the ability to set creation time: alexcrichton/filetime#70. I'm not sure why this is difficult. I am pretty sure creation timestamps are stored the same as last-modified timestamps.

I might try to patch that in the library, but like, 0 chance I know what I'm doing better than alexcrichton. There's probably a reason it's not supported.