This commit changes a minor queue priority design to something which
makes a little bit more sense.

Previously, a song that had already been played would only be
re-enqueued if its priority had just been raised above the current
song's.  This means that if it was already above, it was not
re-enqueued.  That is a surprising behavior, because users expect a
song to be played when its priority is raised.

Now the song is always re-enqueued if its priority is raised (and
above the current song's - no matter if it has already been above
before).

 https://bugs.musicpd.org/view.php?id=4592

e.g. when running 'update-rc.d mpd disable'

https://bugs.musicpd.org/view.php?id=4599

Everything must be S16.

If an error has already been set by f.directory->storage->GetInfo(),
don't set it again.

https://bugs.musicpd.org/view.php?id=4577

It was used in a wrong way, which did not deal with errors
consistently.  And if that's wrong, there is no need for FindStorage()
at all - let's remove it and the confusion around it.

An empty packet would be a command for avcodec_send_packet() to
finalize the codec.

Fixes https://bugs.musicpd.org/view.php?id=4588

By declaring the variable-length array to have a nominal size of 1,
struct TagPoolSlot shrinks from 24 bytes to 16 bytes, because "ref"
and "item" now both fit in one machine word.

The "packed" attribute triggers a clang 4.0 warning, and it's not
necessary.  All we want is correct allocation of this
dynamically-sized struct.

This function exists since FFmpeg 3.1.  Fix a build failure with
FFmpeg 3.0.

These bug reports describe problems with some FFmpeg codecs:

 https://bugs.musicpd.org/view.php?id=4564
 https://bugs.musicpd.org/view.php?id=4568
 https://bugs.musicpd.org/view.php?id=4572

According to the FFmpeg bug tracker, a call to
avcodec_parameters_to_context() is required after
avcodec_alloc_context3():

 https://trac.ffmpeg.org/ticket/5781

This requirement was previously undocumented.

Check {GCC,CLANG}_VERSION==0 or >0 instead of using defined(), which
may render undefined behavior.

GCC_VERSION>0 implies defined(__GNUC__).

This is based on a patch from Ian Scott in 2014. It was never committed,
so I figured I'd fix the outstanding issue and resubmit it.

https://www.mail-archive.com/mpd-devel%40musicpd.org/msg00139.html

Fixes memory leak after stream failure.  See
https://bugs.musicpd.org/view.php?id=4562