demux_{lavf,timeline}: add `--flatten-editions`

Lavf exposes programs/renditions as a flat track list tagged with
program_id. We respect this metadata and group tracks into their
respective programs, as tracks from different programs may be completely
unrelated to each other.

This commits adds an option to flatten the editions and ignore the
grouping.

Fixes: https://github.com/mpv-player/mpv/issues/17836#issuecomment-4345930205