Fix MapLayer functions on derived prop lists (#935)

src/script/C4PropList.cpp

@@ -381,6 +381,18 @@ C4Def const * C4PropList::GetDef() const
 	return 0;
 }
 
+class C4MapScriptLayer * C4PropList::GetMapScriptLayer()
+{
+	if (prototype) return prototype->GetMapScriptLayer();
+	return NULL;
+}
+
+class C4MapScriptMap * C4PropList::GetMapScriptMap()
+{
+	if (prototype) return prototype->GetMapScriptMap();
+	return NULL;
+}
+
 C4PropListNumbered * C4PropList::GetPropListNumbered()
 {
 	if (prototype) return prototype->GetPropListNumbered();

src/script/C4PropList.h

@@ -83,8 +83,8 @@ public:
 	virtual C4Effect * GetEffect();
 	virtual C4PropListNumbered * GetPropListNumbered();
 	C4PropList * GetPrototype() const { return prototype; }
-	virtual class C4MapScriptLayer * GetMapScriptLayer() { return NULL; }
-	virtual class C4MapScriptMap * GetMapScriptMap() { return NULL; }
+	virtual class C4MapScriptLayer * GetMapScriptLayer();
+	virtual class C4MapScriptMap * GetMapScriptMap();
 
 	// saved as a reference to a global constant?
 	virtual class C4PropListStatic * IsStatic() { return NULL; }