Revert commit 70495aad by rewriting it.  Turns out, in old FFmpeg
versions, copying the AVPacket is necessary.

Don't scan tag items if the handler doesn't implement the tag()
method.

This is required if a stream ands without another chained FLAC file.

It's always 0.

If the duration is unknown, pass SignedSongTime::Negative(), as
documented for decoder_initialized().

This is obsolete because it has been moved to the MPD core.

libFLAC API documentation suggests that FLAC__stream_decoder_flush()
should be called to recover from FLAC__STREAM_DECODER_SEEK_ERROR.

Stop after all fatal errors.  This fixes assertion failures in
libFLAC.

Required for OpusTags packets which contain artwork.

See https://bugs.musicpd.org/view.php?id=4520

Reduce some overhead.  It is not necessary to copy the object.

av_free_packet() was deprecated in FFmpeg 3.0.

Fixes http://bugs.musicpd.org/view.php?id=4432

Allocate the buffer dynamically using av_malloc(), and free
AVIOContext.buffer in the destructor, as mandated by the libavformat
documentation.

Fixes http://bugs.musicpd.org/view.php?id=4446