Some sites throttle requests, but throttling can be avoided with many
small range requests. yt-dlp implements a "http_chunk_size" key in its
-J output to inform us of the ideal size.
https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/21703 has just been merged,
which allows use the http_chunk_size to greatly speed up how fast
YouTube videos get loaded into the cache by mpv.
Should the FFmpeg that mpv is built against lack the request_size
AVOption, then this will silently not do anything new, so no version
checks are needed.
Specifying multiple_requests=1 like suggested in
https://github.com/mpv-player/mpv/issues/8655#issuecomment-3872689494
does not seem necessary.
Fixes#8655.
Supersedes #15612.
Co-authored-by: Nicolas F <ovdev@fratti.ch>