ffmpeg: libffmpeg-audio-dec and -custom use integrated opus decoder

ffmpeg: libffmpeg-audio-dec and -custom use integrated opus decoder

For libffmpeg-audio-dec:

Remove @DEVEL flag from libffmpeg-audio-dec. Reasoning for this is
that choosing this package does not rquire any further action by the
builder in what codecs or formats should be built in.
libffmpeg-custom, the other libffmpeg to require @DEVEL, does need
further interaction.

Add @BUILD_PATENTED as at least one of the audio codecs is likely
patent protected.

Add ffmpeg's native Opus decoder to libffmpeg-audio-dec

Remove libopus dependency to libffmpeg-audio-dec as libopus provides
decode and encode support for Opus audio. Since FFmpeg can decode
Opus audio on its own, and libffmpeg-audio-dec is intended for audio
decoding, libopus is no longer required for Opus audio support.

For libffmpeg-custom:

Add in options for ffmpeg's integrated Opus decoder

Signed-off-by: Ian Leonard <redacted>