ao/pulse: properly set device_buffer

Previously, device_buffer defaulted to 0 on pulse. This meant that
commit baa7b5c would always wait with a timeout of 0, leading to
high CPU usage for PulseAudio users.

By setting device_buffer to the number of samples per channel that
PulseAudio sets as its target, this commit fixes this behaviour.

audio/out/ao_pulse.c

@@ -465,6 +465,14 @@ static int init(struct ao *ao)
         goto unlock_and_fail;
     }
 
+    const pa_buffer_attr* final_bufattr = pa_stream_get_buffer_attr(priv->stream);
+    if(!final_bufattr) {
+        MP_ERR(ao, "PulseAudio didn't tell us what buffer sizes it set. Bailing out.\n");
+        goto unlock_and_fail;
+    }
+    ao->device_buffer = final_bufattr->tlength /
+                        af_fmt_to_bytes(ao->format) / ao->channels.num;
+
     pa_threaded_mainloop_unlock(priv->mainloop);
     return 0;