filter/ReplayGain: add method _set_mode()

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

test/run_filter.c

@@ -25,7 +25,6 @@
 #include "pcm_volume.h"
 #include "idle.h"
 #include "mixer_control.h"
-#include "Playlist.hxx"
 #include "stdbin.h"
 
 #include <glib.h>
@@ -35,8 +34,6 @@
 #include <errno.h>
 #include <unistd.h>
 
-struct playlist g_playlist;
-
 void
 idle_add(G_GNUC_UNUSED unsigned flags)
 {

test/run_output.cxx

@@ -32,7 +32,6 @@ extern "C" {
 #include "idle.h"
 }
 
-#include "Playlist.hxx"
 #include "PlayerControl.hxx"
 #include "stdbin.h"
 
@@ -43,8 +42,6 @@ extern "C" {
 #include <unistd.h>
 #include <stdlib.h>
 
-struct playlist g_playlist;
-
 void
 idle_add(G_GNUC_UNUSED unsigned flags)
 {