output/multiple: add "client" field, replacing the "client" parameter

By eliminating GetAnyClient(), we can have instances with no outputs
at all.

src/Main.cxx

@@ -447,8 +447,7 @@ MainConfigured(const struct options &options, const ConfigData &raw_config)
 	for (auto &partition : instance.partitions) {
 		partition.outputs.Configure(instance.rtio_thread.GetEventLoop(),
 					    raw_config,
-					    config.replay_gain,
-					    partition.pc);
+					    config.replay_gain);
 		partition.UpdateEffectiveReplayGainMode();
 	}
 

src/Partition.cxx

@@ -43,7 +43,7 @@ Partition::Partition(Instance &_instance,
 	 idle_monitor(instance.event_loop, BIND_THIS_METHOD(OnIdleMonitor)),
 	 global_events(instance.event_loop, BIND_THIS_METHOD(OnGlobalEvent)),
 	 playlist(max_length, *this),
-	 outputs(*this),
+	 outputs(pc, *this),
 	 pc(*this, outputs,
 	    instance.input_cache.get(),
 	    buffer_chunks,

src/command/PartitionCommands.cxx

@@ -106,8 +106,7 @@ handle_newpartition(Client &client, Request request, Response &response)
 					 ReplayGainConfig());
 	auto &partition = instance.partitions.back();
 	partition.outputs.AddNullOutput(instance.io_thread.GetEventLoop(),
-					ReplayGainConfig(),
-					partition.pc);
+					ReplayGainConfig());
 	partition.UpdateEffectiveReplayGainMode();
 
 	instance.EmitIdle(IDLE_PARTITION);

src/output/MultipleOutputs.cxx

@@ -35,8 +35,9 @@
 #include <assert.h>
 #include <string.h>
 
-MultipleOutputs::MultipleOutputs(MixerListener &_mixer_listener) noexcept
-	:mixer_listener(_mixer_listener)
+MultipleOutputs::MultipleOutputs(AudioOutputClient &_client,
+				 MixerListener &_mixer_listener) noexcept
+	:client(_client), mixer_listener(_mixer_listener)
 {
 }
 
@@ -86,8 +87,7 @@ LoadOutputControl(EventLoop &event_loop,
 void
 MultipleOutputs::Configure(EventLoop &event_loop,
 			   const ConfigData &config,
-			   const ReplayGainConfig &replay_gain_config,
-			   AudioOutputClient &client)
+			   const ReplayGainConfig &replay_gain_config)
 {
 	const AudioOutputDefaults defaults(config);
 	FilterFactory filter_factory(config);
@@ -119,8 +119,7 @@ MultipleOutputs::Configure(EventLoop &event_loop,
 
 void
 MultipleOutputs::AddNullOutput(EventLoop &event_loop,
-			       const ReplayGainConfig &replay_gain_config,
-			       AudioOutputClient &client)
+			       const ReplayGainConfig &replay_gain_config)
 {
 	const AudioOutputDefaults defaults;
 
@@ -147,8 +146,6 @@ void
 MultipleOutputs::Add(std::unique_ptr<FilteredAudioOutput> output,
 		     bool enable) noexcept
 {
-	auto &client = GetAnyClient();
-
 	// TODO: this operation needs to be protected with a mutex
 	outputs.emplace_back(std::make_unique<AudioOutputControl>(std::move(output),
 								  client));

src/output/MultipleOutputs.hxx

@@ -45,6 +45,8 @@ struct ReplayGainConfig;
  * keeps them synchronized.
  */
 class MultipleOutputs final : public PlayerOutputs {
+	AudioOutputClient &client;
+
 	MixerListener &mixer_listener;
 
 	std::vector<std::unique_ptr<AudioOutputControl>> outputs;
@@ -68,17 +70,16 @@ public:
 	 * Load audio outputs from the configuration file and
 	 * initialize them.
 	 */
-	MultipleOutputs(MixerListener &_mixer_listener) noexcept;
+	MultipleOutputs(AudioOutputClient &_client,
+			MixerListener &_mixer_listener) noexcept;
 	~MultipleOutputs() noexcept;
 
 	void Configure(EventLoop &event_loop,
 		       const ConfigData &config,
-		       const ReplayGainConfig &replay_gain_config,
-		       AudioOutputClient &client);
+		       const ReplayGainConfig &replay_gain_config);
 
 	void AddNullOutput(EventLoop &event_loop,
-			   const ReplayGainConfig &replay_gain_config,
-			   AudioOutputClient &client);
+			   const ReplayGainConfig &replay_gain_config);
 
 	/**
 	 * Returns the total number of audio output devices, including
@@ -156,10 +157,6 @@ public:
 	void SetSoftwareVolume(unsigned volume) noexcept;
 
 private:
-	AudioOutputClient &GetAnyClient() noexcept {
-		return outputs.front()->GetClient();
-	}
-
 	/**
 	 * Was Open() called successfully?
 	 *