Add filename support for --playlist-start

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https://github.com/mpv-player/mpv/labels/meta%3Afeature-request

Expected behavior of the wanted feature

--playlist-start only supports index and auto, but when I open a playlist, I need the playlist to be sorted by mtime. I can use ls -t | mpv --playlist=- to do that, but I need to start from the specific file instead of the first one if not providing index for --playlist-start, fetching the index is not easy, passing the file to it is easier.

Most of the time I use mpv to open a file inside ranger using Enter or Right key on a video file, the files list in the directory are sorted by mtime, and I hope to open a video file and the playlist is sorted by mtime like my ranger does.

Changing the line in ~/.config/ranger/rifle.conf:

mime ^video,        has mpv,      X, flag f = mpv -- "$@"

to

mime ^video,        has mpv,      X, flag f = /bin/ls -t | mpv --playlist=- -- "$@"

is ok for playlist to be sorted my mtime, but mpv won't play the video file when I hit Enter or Right key, and --playlist-start=10 doesn't seem to work.

If anyone can get an easy solution instead implementing filename for --playlist-start, it would be also OK.

Alternative behavior of the wanted feature

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Reproduce the reported behavior using --playlist-start and the ranger ~/.config/ranger/rifle.conf command, with a playlist sorted by mtime. No source file or test is identified in the issue; done should mean that opening a selected file starts the sorted playlist at that filename, or that an explicitly documented alternative achieves the same result.