output/Internal: add OpenSoftwareMixer()

src/output/Internal.cxx

@@ -99,6 +99,13 @@ AudioOutput::CloseOutput(bool drain) noexcept
 }
 
 void
+AudioOutput::OpenSoftwareMixer() noexcept
+{
+	if (mixer != nullptr && mixer->IsPlugin(software_mixer_plugin))
+		software_mixer_set_filter(*mixer, volume_filter.Get());
+}
+
+void
 AudioOutput::CloseSoftwareMixer() noexcept
 {
 	if (mixer != nullptr && mixer->IsPlugin(software_mixer_plugin))

src/output/Internal.hxx

@@ -163,6 +163,11 @@ public:
 	/**
 	 * Mutex must not be locked.
 	 */
+	void OpenSoftwareMixer() noexcept;
+
+	/**
+	 * Mutex must not be locked.
+	 */
 	void CloseSoftwareMixer() noexcept;
 
 	void BeginPause() noexcept;

src/output/Thread.cxx

@@ -26,7 +26,6 @@
 #include "notify.hxx"
 #include "filter/plugins/ConvertFilterPlugin.hxx"
 #include "mixer/MixerInternal.hxx"
-#include "mixer/plugins/SoftwareMixerPlugin.hxx"
 #include "thread/Util.hxx"
 #include "thread/Slack.hxx"
 #include "thread/Name.hxx"
@@ -53,10 +52,7 @@ AudioOutputControl::InternalOpen2(const AudioFormat in_audio_format)
 {
 	assert(in_audio_format.IsValid());
 
-	if (output->mixer != nullptr &&
-	    output->mixer->IsPlugin(software_mixer_plugin))
-		software_mixer_set_filter(*output->mixer,
-					  output->volume_filter.Get());
+	output->OpenSoftwareMixer();
 
 	const auto cf = in_audio_format.WithMask(output->config_audio_format);