add video (re)-encoding option

[sfmig]

The user may want to compress the video data or re-encode them (see the comment here). This could be part of the uploading/metadata addition stage.

[adamltyson]

A very common issue is that videos are not reliably seekable, meaning that you may not get the same data when trying to read a particular frame index.

Well that's terrifying

[sfmig]

I know, I was not aware of this potential issue

[niksirbi]

I like having this option. It means that ffmpeg would be a dependency, but you would anyway need that in most cases when working with videos.