Add code to extract sla supports outline for arrangement

Add functionality to libnest to support on_packed handler for all items.

Use it to swap arrange polygon from envelope to core poly after packing was succesful.
Solving brim issue by offset adjustments of arrange polygons

Only in sla yet

src/slic3r/GUI/Jobs/ArrangeJob.hpp

@@ -21,6 +21,8 @@ class ArrangeJob : public Job
 
     ArrangePolygons m_selected, m_unselected, m_unprintable;
     std::vector<ModelInstance*> m_unarranged;
+    coord_t m_min_inflation = 0;
+
     Plater *m_plater;
 
     // clear m_selected and m_unselected, reserve space for next usage