Add a hidden "Fade" property that modifies the effective opacity to enable other functionality (closes #1970)

include/openspace/rendering/renderable.h

@@ -105,6 +105,7 @@ public:
 protected:
     properties::BoolProperty _enabled;
     properties::FloatProperty _opacity;
+    properties::FloatProperty _fade;
     properties::StringProperty _renderableType;
 
     void setBoundingSphere(double boundingSphere);
@@ -113,6 +114,9 @@ protected:
     void setRenderBinFromOpacity();
     void registerUpdateRenderBinFromOpacity();
 
+    /// Returns the full opacity constructed from the _opactiy and _fade property values
+    float opacity() const;
+
     double _boundingSphere = 0.0;
     double _interactionSphere = 0.0;
     SceneGraphNode* _parent = nullptr;