Fix for overlapping lines when extra wall lines > 0
Fix for overlapping lines when extra wall lines > 0

The cut of the denser area now also performed when zig_zaggify is false.
This ensures that the infill lines of denser areas don't overlap with
the wall generated by the least dense area.

Also simplified cutting logic and speed-up. since the infill computation
always gets an empty Polygon and doesn't have to recalculate the previous
crossings.

Contributes to CURA-7115 and fixes Ultimaker/Cura#8628

Ultimaker/CuraEngine,Ultimaker/CuraEngine