- split DObject::Destroy into the main method, a native OnDestroy and a scripted OnDestroy method and made the main method non-virtual

This was done to ensure it can be properly overridden in scripts without causing problems when called during engine shutdown for the type and symbol objects the VM needs to work and to have the scripted version always run first.
Since the scripted OnDestroy method never calls the native version - the native one is run after the scripted one - this can be simply skipped over during shutdown.

src/fragglescript/t_func.cpp

@@ -1786,7 +1786,7 @@ public:
 	DLightLevel(sector_t * s,int destlevel,int speed);
 	void	Serialize(FSerializer &arc);
 	void		Tick ();
-	void		Destroy() { Super::Destroy(); m_Sector->lightingdata=NULL; }
+	void		OnDestroy() { Super::OnDestroy(); m_Sector->lightingdata = nullptr; }
 };
 
 IMPLEMENT_CLASS(DLightLevel, false, false)