Assigning nullptr to next_song may disrupt the player thread and
render undefined behavior.

Move ResetCrossFade() to where cross-fading would actually break, to
make the cross-fading state more reliable.

This new library is integrated in the PcmExport class and (if enabled)
converts MPD's channel order (= FLAC channel order) to ALSA channel
order.

This fixes:
 http://bugs.musicpd.org/view.php?id=3147
and
 http://bugs.musicpd.org/view.php?id=3255

Basic definition to solve http://bugs.musicpd.org/view.php?id=3147 and
http://bugs.musicpd.org/view.php?id=3255

This systemd feature has been removed a while ago without replacement,
and it turns out that systemd developers suggest not using control
groups at all to assign real-time privileges.  Therfore, a replacement
feature will not be implement in future systemd releases, and we can
really remove those lines completely.

See http://bugs.musicpd.org/view.php?id=4413

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

Fixes mixer lag (http://bugs.musicpd.org/view.php?id=4425).

This reverts commit d7d9dbd2 by
reimplementing it with the current MPD API.

3 years ago, I was wrong about the "embcue" plugin being able to
replace this one, because "embcue" reads a tag named "CUESHEET", while
this plugin reads the "CUESHEET" FLAC metablock.  There's an important
difference between those two!

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

Avoids the recursion in LoadFile().

Avoid duplicate AllocatedPath::FromUTF8() invocations in two callers.

Allocate the frame pointers only when actually decoding the file.