- separated class descriptors from VM types.

Combining these two groups of data has been the cause of many hard to detect errors because it allowed liberal casting between types that are used for completely different things.

src/scripting/thingdef_properties.cpp

@@ -92,7 +92,7 @@ static PClassActor *FindClassTentative(const char *name, PClass *ancestor, bool
 	}
 	if (cls->Size == TentativeClass && optional)
 	{
-		cls->ObjectFlags |= OF_Transient;	// since this flag has no meaning in class types, let's use it for marking the type optional.
+		cls->bOptional = true;
 	}
 	return static_cast<PClassActor *>(cls);
 }