Fixed an issue, where the output G-code file name was not always updated

from the current Model/ModelObjects.
Fixed a possible race condition in updating Print::m_placeholder_parser
with the proposed filename / filename base.
Improved documentation (source code comments).

src/libslic3r/SLAPrint.cpp

@@ -387,8 +387,6 @@ SLAPrint::ApplyStatus SLAPrint::apply(const Model &model, const DynamicPrintConf
             update_apply_status(false);
     }
 
-    this->update_object_placeholders();
-
 #ifdef _DEBUG
     check_model_ids_equal(m_model, model);
 #endif /* _DEBUG */