mixer/Memento: move IDLE_MIXER out of SetVolume()

src/command/OtherCommands.cxx

@@ -338,6 +338,7 @@ handle_setvol(Client &client, Request args, Response &)
 
 	auto &partition = client.GetPartition();
 	partition.mixer_memento.SetVolume(partition.outputs, level);
+	partition.EmitIdle(IDLE_MIXER);
 	return CommandResult::OK;
 }
 
@@ -362,8 +363,10 @@ handle_volume(Client &client, Request args, Response &r)
 	else if (new_volume > 100)
 		new_volume = 100;
 
-	if (new_volume != old_volume)
+	if (new_volume != old_volume) {
 		mixer_memento.SetVolume(outputs, new_volume);
+		partition.EmitIdle(IDLE_MIXER);
+	}
 
 	return CommandResult::OK;
 }

src/mixer/Memento.cxx

@@ -68,8 +68,6 @@ MixerMemento::SetVolume(MultipleOutputs &outputs, unsigned volume)
 
 	volume_software_set = volume;
 
-	idle_add(IDLE_MIXER);
-
 	SetHardwareVolume(outputs, volume);
 }
 

src/mixer/Memento.hxx

@@ -49,6 +49,8 @@ public:
 
 	/**
 	 * Throws on error.
+	 *
+	 * Note: the caller is responsible for emitting #IDLE_MIXER.
 	 */
 	void SetVolume(MultipleOutputs &outputs, unsigned volume);