demux_playlist: extend maximum line size

Raise it from 8KB to 512KB.

Do this because ytdl_hook.lua generated a 40KB EDL file (from 80KB
youtube-dl JSON output), and putting it into a .m3u file for easier
debugging failed due to the size limit.

demux/demux_playlist.c

@@ -47,7 +47,7 @@ static bool check_mimetype(struct stream *s, const char *const *list)
 struct pl_parser {
     struct mp_log *log;
     struct stream *s;
-    char buffer[8 * 1024];
+    char buffer[512 * 1024];
     int utf16;
     struct playlist *pl;
     bool error;