Fix skirt and brim handling for duplicated objects (instances) (#14683)

src/libslic3r/ObjectID.hpp

@@ -3,6 +3,7 @@
 
 #include <cereal/access.hpp>
 #include <cereal/types/base_class.hpp>
+#include <cstddef>
 
 namespace Slic3r {
 
@@ -41,6 +42,18 @@ private:
 	template<class Archive> void serialize(Archive &ar) { ar(id); }
 };
 
+struct ObjectInstanceID {
+    ObjectID object_id;
+    size_t   instance_id { size_t(-1) };
+
+    bool operator==(const ObjectInstanceID& rhs) const { return object_id == rhs.object_id && instance_id == rhs.instance_id; }
+    bool operator!=(const ObjectInstanceID& rhs) const { return !(*this == rhs); }
+    bool operator<(const ObjectInstanceID& rhs) const
+    {
+        return object_id < rhs.object_id || (object_id == rhs.object_id && instance_id < rhs.instance_id);
+    }
+};
+
 // Base for Model, ModelObject, ModelVolume, ModelInstance or ModelMaterial to provide a unique ID
 // to synchronize the front end (UI) with the back end (BackgroundSlicingProcess / Print / PrintObject).
 // Also base for Print, PrintObject, SLAPrint, SLAPrintObject to provide a unique ID for matching Model / ModelObject