SGCT compile fix

First pass for pull request

include/openspace/properties/property.h

@@ -500,7 +500,7 @@ public:
      *
      * \return true if the property has changed
      */
-    bool hasChanged();
+    bool hasChanged() const;
 
     /**
      * Reset the valChanged flag to an unchanged state, as if value has not been changed.
@@ -544,7 +544,7 @@ protected:
     std::vector<std::pair<OnDeleteHandle, std::function<void()>>> _onDeleteCallbacks;
 
     /// Flag indicating that this property value has been changed after initialization
-    bool valChanged = false;
+    bool _isValueDirty = false;
 
 private:
     void notifyDeleteListeners();

include/openspace/properties/templateproperty.inl

@@ -176,7 +176,7 @@ void openspace::properties::TemplateProperty<T>::setValue(T val) {
     if (val != _value) {
         _value = std::move(val);
         notifyChangeListeners();
-        valChanged = true;
+        _isValueDirty = true;
     }
 }
 
@@ -197,7 +197,7 @@ void TemplateProperty<T>::set(std::any value) {
     if (v != _value) {
         _value = std::move(v);
         notifyChangeListeners();
-        valChanged = true;
+        _isValueDirty = true;
     }
 }