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

This bug caused a 9 second offset in all time stamps.  Due to that,
the Snapcast server thought the chunks are too old and discarded them.

Fixes https://github.com/MusicPlayerDaemon/MPD/discussions/1287

Don't allocate any memory until the output is really enabled.

The implementation was removed 12 years ago in commit f6455d5f and
nobody missed it.