A bit of a long standing pain with scripting is that when opening a file
that gets interpreted as a playlist (like an m3u), the original path of
the file gets thrown away later. Workarounds basically consist of
getting the filename before mpv expands the path, but that's not really
reliable. Instead of throwing it away, save the original playlist path
by copying to the playlist entries when applicable (demuxer playlist and
the playlist option). Then expose these as properties: playlist-path for
the currently playing entry and playlist/N/playlist-path for each
specific entry. Closes#8508, #7605.
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