FIX: fitst layer bbox was wrong for multiple plates

brim bounding box is now shifted w.r.t. plate origin, need to modify the
no-brim shift the same way.

Change-Id: Iefa7287e2848aadbfc4a468bca394ecec48a1e7c

src/libslic3r/Print.hpp

@@ -201,6 +201,9 @@ struct PrintInstance
     // 
     // instance id
     size_t               id;
+
+    //BBS: instance_shift is too large because of multi-plate, apply without plate offset.
+    Point shift_without_plate_offset() const;
 };
 
 typedef std::vector<PrintInstance> PrintInstances;