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.

test/run_output.c

@@ -26,6 +26,7 @@
 #include "filter_registry.h"
 #include "pcm_convert.h"
 #include "event_pipe.h"
+#include "idle.h"
 
 #include <glib.h>
 
@@ -33,6 +34,11 @@
 #include <string.h>
 #include <unistd.h>
 
+void
+idle_add(G_GNUC_UNUSED unsigned flags)
+{
+}
+
 void
 event_pipe_emit(G_GNUC_UNUSED enum pipe_event event)
 {