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>

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

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

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

SonarLint reports the latter to be better:

std::scoped_lock basically provides the same feature as std::lock_guard,
but is more generic: It can lock several mutexes at the same time, with a
deadlock prevention mechanism (see {rule:cpp:S5524}). The equivalent code
to perform simultaneous locking with std::lock_guard is significantly more
complex. Therefore, it is simpler to use std::scoped_lock all the time,
even when locking only one mutex (there will be no performance impact).
Signed-off-by: Rosen Penev <rosenp@gmail.com>

C arrays can be used with make_unique in C++17.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

This fixes this build error observed with clang and -stdlib=libc++:

../mpd-0.23.3/src/output/plugins/PipeWireOutputPlugin.cxx:661:55: error: implicit instantiation of undefined template 'std::array<std::byte, 64>'
        std::array<std::byte, MAX_CHANNELS * MAX_INTERLEAVE> buffer;
                                                             ^
/usr/include/c++/v1/__tuple:219:64: note: template is declared here
template <class _Tp, size_t _Size> struct _LIBCPP_TEMPLATE_VIS array;
                                                               ^

With the "wave" encoder, this has no effect, but it's more correct.

This uncomments the code which had been present already in the first
Snapcast commit (copied from the "httpd" output plugin), but I
commented it because I did not know whether I needed to send silence
samples to all Snapcast clients.

As a side effect, this fixes playback when no Snapcast client is
connected; this was broken because Pause() always returned a positive
value when there were no clients.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1310

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1297

MPD supports only 8 channels, so MAX_CHANNELS is enough, the array
doens't need to be SPA_AUDIO_MAX_CHANNELS (which is 64).

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1297