output/PipeWire: after Cancel(), refill buffer before resuming playback

Deactivate the stream in Cancel().  This fixes stuttering after a
manual song change by refilling the whole ring buffer before
reactivating the stream.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1354

NEWS

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

src/output/plugins/PipeWireOutputPlugin.cxx

@@ -901,6 +901,15 @@ PipeWireOutput::Cancel() noexcept
 	/* clear libpipewire's buffer */
 	pw_stream_flush(stream, false);
 	drained = true;
+
+	/* pause the PipeWire stream so libpipewire ceases invoking
+	   the "process" callback (we have no data until our Play()
+	   method gets called again); the stream will be resume by
+	   Play() after the ring_buffer has been refilled */
+	if (active) {
+		active = false;
+		pw_stream_set_active(stream, false);
+	}
 }
 
 bool