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

Requires PipeWire 0.3.32.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1283