output/PipeWire: call pw_stream_flush() in Cancel()

Clear not only MPD's ring buffer, but also libpipewire's buffers, to
avoid playing some audio from the previous song after a manual song
change.

Fixes part 1 of https://github.com/MusicPlayerDaemon/MPD/issues/1354

NEWS

@@ -6,6 +6,7 @@ ver 0.23.8 (not yet released)
   - cdio_paranoia: faster cancellation
   - cdio_paranoia: don't scan for replay gain tags
   - pipewire: fix playback of very short tracks
+  - pipewire: drop all buffers before manual song change
   - snapcast: fix busy loop while paused
   - snapcast: fix stuttering after resuming playback
 * mixer

src/output/plugins/PipeWireOutputPlugin.cxx

@@ -895,7 +895,12 @@ PipeWireOutput::Cancel() noexcept
 	if (drained)
 		return;
 
+	/* clear MPD's ring buffer */
 	ring_buffer->reset();
+
+	/* clear libpipewire's buffer */
+	pw_stream_flush(stream, false);
+	drained = true;
 }
 
 bool