Fix some rotation-related issues (#3890)

* Fix issue with max_z after rotation

* Don't use `object.bounding_box()`, use `bounding_box_exact()` instead.

* Fix auto-orient

---------

Co-authored-by: SoftFever <softfeverever@gmail.com>

src/libslic3r/ModelArrange.cpp

@@ -176,7 +176,7 @@ ArrangePolygon get_instance_arrange_poly(ModelInstance* instance, const Slic3r::
         ap.has_tree_support = true;
     }
 
-    ap.height = obj->bounding_box().size().z();
+    ap.height = obj->bounding_box_exact().size().z();
     ap.name = obj->name;
     return ap;
 }