replay_gain: reimplement as a filter plugin

Apply the replay gain in the output thread.  This means a new setting
will be active instantly, without going through the whole music pipe.
And we might have different replay gain settings for each audio output
device.

src/chunk.c

@@ -29,6 +29,7 @@ music_chunk_init(struct music_chunk *chunk)
 {
 	chunk->length = 0;
 	chunk->tag = NULL;
+	chunk->replay_gain_serial = 0;
 }
 
 void