From Evgeni:

To get rid of tiny gaps in iso-lines drawing  on top surface of the frame I’ve
introduced new method TPainter3dAlgorithms::DrawLevelLines()
that draws iso-lines without applying hidden line removal.

In addition declaration of methods in TPainter3dAlgorithms.h have been
partially regrouped to have better structure of the class definition. Some
methods are now declared as protected.