Closes https://github.com/MusicPlayerDaemon/MPD/issues/1721

It is not explicitly documented whether snd_pcm_name() is allowed to
return NULL:
https://www.alsa-project.org/alsa-doc/alsa-lib/group___p_c_m.html#ga5031edc0422df8db1f70af056a12dd77

But apparently this is legal:
https://github.com/alsa-project/alsa-lib/blob/0222f45d11e8b71bf651b985b00fdb0addbf3eed/src/pcm/pcm.c#L2761-L2762

That's ... surprising!

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1645

Eliminate global variables, convert them to MixerMemento fields.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1583

This PR provides forward and backward compatibility at macos SDK 12.0. At SDK 12.0, API function names were changed essentially replacing
occurrences of the word Master/master with Main/main. This change was test built on two different systems.

1. macos 10.15.7 with Xcode 12.4 and clang 12.0.0 on x86_64
2. macos 12.5 with Xcode 13.4.1 and clang 13.1.6 on arm64 (Apple silicon M1)

It should be noted that on macos 10.15.7 with Xcode 11.2 and clang 11.0, MPD will not build.
The MPD documentation states that clang 11.0 is the minimum requirement,
but clang 11.0 produces compile errors. Apparently the macos version
of clang 11.0 is not fully compliant.

Closes #1564

Deactivate the stream in Cancel().  This fixes stuttering after a
manual song change by refilling the whole ring buffer before
reactivating the stream.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1354

Clear not only MPD's ring buffer, but also libpipewire's buffers, to
avoid playing some audio from the previous song after a manual song
change.

Fixes part 1 of https://github.com/MusicPlayerDaemon/MPD/issues/1354

Removing the LockHasClients(); this code was copied from the "httpd"
output plugin, but unlike "httpd", the SnapCast output plugin does not
feed silence while paused, so we need to implement a delay to avoid
busy-looping the CPU.

As a side effect, this eliminates the suttering after resuming
playback, because the timer now gets reset even if there is a client.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1394

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1484

After connecting, PipeWire sometimes sends SPA_PROP_channelVolumes
with no values, and this led to "volume=-NaN".

Since PipeWire 0.3.53, there is no control name anymore, therefore the
name check doesn't work anymore, breaking volume change events.

This obsoletes the crash bug fix in commit 2ee57f9b

Fixes a potential crash bug.

This is a lower-level function without some of the clutter of
PipeWireOutput::SetVolume() which is not needed in that case.

If the PipeWire output has not yet been enabled and no thread_loop has
been created yet, a nullptr dereference in SetVolume() was possible
because nullptr was passed to pw_thread_loop_lock().

This reveals more about the nature of an error instead of just
returning "problems setting volume".

Since PipeWire 0.3.53, control names can apparently be nulled, leading
to crashes in applications assertion that the string cannot be
nullptr.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1558

Even if the "wave_encoder" option is disabled (and no other encoder
plugins are enabled), forcefully enable the Wave encoder (if Snapcast
is enabled).

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1500

Previous versions of MPD would call SetVolume on enabled outputs before
they are ready, causing all of MPD to crash. Checking the really_enabled
flag prevents this, though it also prevents setting volume before the
player starts.

Before (with the PipeWire output):
  [i] ~$ mpc clear
  volume: 81%   repeat: off   random: off   single: off   consume: off
  [i] ~$ systemctl --user restart mpd.service
  [i] ~$ mpc volume 100
  MPD error: Connection closed by the server
  [i] ~ 1 $

After:
  [i] ~$ # mpd is freshly started w/o anything in the queue
  [i] ~$ mpc
  volume:100%   repeat: off   random: off   single: off   consume: off
  [i] ~$ mpc volume 80
  MPD error: problems setting volume
  [i] ~ 1 $ mpc
  volume:100%   repeat: off   random: off   single: off   consume: off
  [i] ~$

Previous versions of MPD would, on parameter change, set the PipeWire
volume before clearing the restore_volume flag, causing the call to
short circuit and do nothing. Instead, clear the flag before the call.

This version was released 7 years ago, and it's reasonable to require
at least this version.

Fixes -Wunused-result

Wasapi output plugin won't start playing after being paused

The cause is that the scope guard in the WASAPI work thread
(WasapiOutputPlugin.cxx, function WasapiOutputThread::Work(), in the
while (true) loop) is set up too 'late' in the execution. There is one
condition ("if (data_in_frames >= buffer_size_in_frames)") when it is
hit, the loop will continue without executing the scope guard. This
scope guard is responsible for emptying the buffer again, and if the
buffer is not emptied, the above mentioned condition will stay true.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1451

Must be initialized for the check in SetVolume().

Signed-off-by: Rosen Penev <rosenp@gmail.com>