mixer_plugin: pass audio_output pointer to mixer_plugin.init()

This allows the mixer object to access its associated audio output
object.

src/mixer/alsa_mixer_plugin.c

@@ -52,7 +52,7 @@ alsa_mixer_quark(void)
 }
 
 static struct mixer *
-alsa_mixer_init(const struct config_param *param,
+alsa_mixer_init(G_GNUC_UNUSED void *ao, const struct config_param *param,
 		G_GNUC_UNUSED GError **error_r)
 {
 	struct alsa_mixer *am = g_new(struct alsa_mixer, 1);

src/mixer/oss_mixer_plugin.c

@@ -74,7 +74,8 @@ oss_find_mixer(const char *name)
 }
 
 static struct mixer *
-oss_mixer_init(const struct config_param *param, GError **error_r)
+oss_mixer_init(G_GNUC_UNUSED void *ao, const struct config_param *param,
+	       GError **error_r)
 {
 	struct oss_mixer *om = g_new(struct oss_mixer, 1);
 

src/mixer/pulse_mixer_plugin.c

@@ -212,7 +212,7 @@ context_state_cb(pa_context *context, void *userdata)
 
 
 static struct mixer *
-pulse_mixer_init(const struct config_param *param,
+pulse_mixer_init(G_GNUC_UNUSED void *ao, const struct config_param *param,
 		 G_GNUC_UNUSED GError **error_r)
 {
 	struct pulse_mixer *pm = g_new(struct pulse_mixer,1);

src/mixer/software_mixer_plugin.c

@@ -37,7 +37,8 @@ struct software_mixer {
 };
 
 static struct mixer *
-software_mixer_init(G_GNUC_UNUSED const struct config_param *param,
+software_mixer_init(G_GNUC_UNUSED void *ao,
+		    G_GNUC_UNUSED const struct config_param *param,
 		    G_GNUC_UNUSED GError **error_r)
 {
 	struct software_mixer *sm = g_new(struct software_mixer, 1);