Fix crash previewing certain smudge-lightness brushes
Under certain circumstances, loading a smudge brush with a lightness brush tip with the preset editor open, or switching a smudge brush to a lightness brush tip, will cause Krita to crash. Apparently some brushes will load without having UseNewEngine set, and switching to Lightness won't always set it in the KisPaintOpSettings.
BUG 439127