Add "Cancel Objects" support for Prusa/Marlin/RRF firmware (#3164)

object cancel support for Prusa/Marlin/RRF

src/libslic3r/Print.hpp

@@ -204,6 +204,8 @@ struct PrintInstance
     // 
     // instance id
     size_t               id;
+    // Orca: unique id used by marlin/rrf cancel object feature
+    size_t               unique_id;
 
     //BBS: instance_shift is too large because of multi-plate, apply without plate offset.
     Point shift_without_plate_offset() const;