bugfix: Make OffsetNode behave as AbstractPolyNode if a visitor doesn't handle it

src/visitor.h

@@ -53,7 +53,7 @@ public:
 		return visit(state, (const class AbstractNode &)node);
 	}
   virtual Response visit(class State &state, const class OffsetNode &node) {
-		return visit(state, (const class AbstractNode &)node);
+		return visit(state, (const class AbstractPolyNode &)node);
 	}
 	// Add visit() methods for new visitable subtypes of AbstractNode here
 };