Helps prevent / minimize issues with filament droop.
Expecially when Cura decides to start the top of hole in mid-air.
Visible Changes:
* Supportless screw holes have a 3rd layer.
* Supportless magnet holes without screw holes have a 3rd layer.
Backend Changes:
* Switched to a completely different generation strategy.
* Previous strategy directly produced negative.
* New strategy is to make a positive,
then use that to create a negative.
* Algorithm for multiple layers is not perfect,
but works within tollerances set.
