replay_gain: optionally use hardware mixer to apply replay gain

Add an option for each audio output which enables the use of the
hardware mixer, instead of the software volume code.

This is hardware specific, and assumes linear volume control.  This is
not the case for hardware mixers which were tested, making this patch
somewhat useless, but we will use it to experiment with the settings,
to find a good solution.

test/run_filter.c

@@ -24,6 +24,7 @@
 #include "filter_plugin.h"
 #include "pcm_volume.h"
 #include "idle.h"
+#include "mixer_control.h"
 
 #include <glib.h>
 
@@ -37,6 +38,13 @@ idle_add(G_GNUC_UNUSED unsigned flags)
 {
 }
 
+bool
+mixer_set_volume(G_GNUC_UNUSED struct mixer *mixer,
+		 G_GNUC_UNUSED unsigned volume, G_GNUC_UNUSED GError **error_r)
+{
+	return true;
+}
+
 static void
 my_log_func(const gchar *log_domain, G_GNUC_UNUSED GLogLevelFlags log_level,
 	    const gchar *message, G_GNUC_UNUSED gpointer user_data)