"Depth first" carving optimization

Hi @MiguelAngelCottoMora, @TravisLucia, @Mike, @Alex1380,

Latest changes are online now. Please have a look at "Depth first" carving for nested shapes

During this optimisation we had to re-implement part of our layering approach and for a while nested outlines were done layer-by-layer. We apologize for any inconvenience.

Thank you

Ruwan.