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

Right after booting, the monotonic clock starts with a very small
value, and AudioOutput::LockUpdate() may believe that the fail_timer
has not recovered yet.

Return false on error, not nullptr.

There are code paths (mostly error cases) in which it is possible to
initialize an AudioOutput and then kill it without ever calling
audio_output_new().  In such a case, its destructor will attempt to
free a mixer that was never initialized, leading to an attempt to
take out a lock on a mutex that was similarly never initialized,
which hangs forever.

Fix by always initializing the mixer appropriately.

A mutex acts as a memory barrier, and thus "volatile" is not
necessary.

Works around the "PCM conversion from f to dsd is not implemented"
error message that prevents DSD playback.

Fixes memory leak because ao_plugin_close() never gets called.

Be consistent.

A mutex acts as a memory barrier, and thus "volatile" is not
necessary.

The Release Track Id uniquely identifies a recording on a release - that
is, even if a recording appears twice on a release (meaning that the
combination of recording and release id are not enough to figure out
which one it is), the release track id will allow differentiating the two.

The tag names are taken from
https://musicbrainz.org/doc/MusicBrainz_Picard/Tags/Mapping

Eliminate the duplicate pa_operation_get_state() call.

The standard has been renamed since the early draft that was
implemented in MPD.

Necessary for "final" on gcc 4.6.

Translate SampleFormat::DSD to SND_PCM_FORMAT_DSD_U8, which was added
to alsa-lib 1.0.27.1.

API simplification.  We can now avoid abusing a "size_t*" as
additional return value.

Enables using range-based "for".

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>

-fixes SIGABRT when mpd is started from a previously paused state
-regression introduced by commit: f5a923b9:
'OutputAll: convert to class, move instance to class Partition'

-input_audio_format was previously declared using the static modifier
ensuring it was zero-initialised by default
-the current default-initialised input_audio_format contains garbage at
runtime which allows the AudioFormat.IsDefined() 'fail fast' test in
MultipleOutputs::Update to pass and the SIGABRT follows in
AudioOutput::Open when passed the invalid input_audio_format struct
-switching AudioFormat.IsDefined() for AudioFormat.IsValid() is an
alternative workaround

Has been superseded by the previous commit.