filter/ReplayGain: add method _set_mode()

Push the new mode to the filter instead of accessing global variables
through replay_gain_get_real_mode().

src/OutputControl.hxx

@@ -20,6 +20,8 @@
 #ifndef MPD_OUTPUT_CONTROL_HXX
 #define MPD_OUTPUT_CONTROL_HXX
 
+#include "replay_gain_info.h"
+
 #include <glib.h>
 
 #include <stddef.h>
@@ -36,6 +38,10 @@ audio_output_quark(void)
 	return g_quark_from_static_string("audio_output");
 }
 
+void
+audio_output_set_replay_gain_mode(struct audio_output *ao,
+				  enum replay_gain_mode mode);
+
 /**
  * Enables the device.
  */