FFmpeg IEC 61937 packetizer

This libmpv build incorporates FFmpeg 6.0's libavformat/spdifenc.c,
libavformat/spdif.c and libavcodec/dca_sample_rate_tab.h, with private
symbol names and an AVIO padding bridge supplied by Zyryn.

Copyright (c) 2009 Bartlomiej Wolowiec
Copyright (c) 2010, 2020 Anssi Hannula
Copyright (c) 2010 Carl Eugen Hoyos
Copyright (C) 2004 Gildas Bazin
Copyright (C) 2004 Benjamin Zores
Copyright (C) 2006 Benjamin Larssonb
Copyright (C) 2007 Konstantin Shishkov

Licensed under GNU LGPL version 2.1 or later. See COPYING.LGPL-2.1.txt.
Original source: https://ffmpeg.org/releases/ffmpeg-6.0.tar.xz
The corresponding Zyryn bridge and reproducible build recipe are
native/audio/mpv_spdif.c and scripts/audio/build_mpv_spdif.sh.
