Fixed regression introduced by the recent PerimeterGenerator refactoring causing spiral vase not to be correctly skipped on multi-loop layers. Includes regression test. #2761

xs/src/libslic3r/ExtrusionEntity.hpp

@@ -35,6 +35,9 @@ enum ExtrusionLoopRole {
 class ExtrusionEntity
 {
     public:
+    virtual bool is_collection() const {
+        return false;
+    };
     virtual bool is_loop() const {
         return false;
     };