Keep master bus color on FX Mixer when switching project

Fixes #6398

include/Mixer.h

@@ -31,7 +31,7 @@
 #include "ThreadableJob.h"
 
 #include <atomic>
-
+#include <optional>
 #include <QColor>
 
 namespace lmms
@@ -76,18 +76,13 @@ class MixerChannel : public ThreadableJob
 		bool requiresProcessing() const override { return true; }
 		void unmuteForSolo();
 
-
 		void setColor (QColor newColor)
 		{
 			m_color = newColor;
-			m_hasColor = true;
 		}
 
-		// TODO C++17 and above: use std::optional instead
-		QColor m_color;
-		bool m_hasColor;
+		std::optional<QColor> m_color;
 
-	
 		std::atomic_int m_dependenciesMet;
 		void incrementDeps();
 		void processed();

include/Track.h

@@ -32,6 +32,7 @@
 #include "AutomatableModel.h"
 #include "JournallingObject.h"
 #include "lmms_basics.h"
+#include <optional>
 
 
 namespace lmms
@@ -191,11 +192,11 @@ public:
 	
 	QColor color()
 	{
-		return m_color;
+		return m_color.value();
 	}
 	bool useColor()
 	{
-		return m_hasColor;
+		return m_color.has_value();
 	}
 
 	bool isMutedBeforeSolo() const
@@ -241,8 +242,7 @@ private:
 
 	QMutex m_processingLock;
 	
-	QColor m_color;
-	bool m_hasColor;
+	std::optional<QColor> m_color;
 
 	friend class gui::TrackView;