Preserve invalid metatile selections

include/ui/metatileselector.h

@@ -50,6 +50,7 @@ private:
     QList<uint16_t> *externalSelectedMetatiles;
 
     void updateSelectedMetatiles();
+    void updateExternalSelectedMetatiles();
     uint16_t getMetatileId(int x, int y);
     QPoint getMetatileIdCoords(uint16_t);
     bool shouldAcceptEvent(QGraphicsSceneMouseEvent*);