ao/pulse: implement period_size

The idea behind period_size is that it's the minimum amount of data
that your audio subsystem wants to fetch.

For PulseAudio, this is given by the minreq buffer attribute, which
is in bytes for all channels. Hence the divisions.

audio/out/ao_pulse.c

@@ -473,6 +473,7 @@ static int init(struct ao *ao)
     }
     ao->device_buffer = final_bufattr->tlength /
                         af_fmt_to_bytes(ao->format) / ao->channels.num;
+    ao->period_size = final_bufattr->minreq / af_fmt_to_bytes(ao->format) / ao->channels.num;
 
     pa_threaded_mainloop_unlock(priv->mainloop);
     return 0;