limited anchors, while before a full perimeter segment was always
taken if possible.
2) Adapted the line infills (grid, stars, triangles, cubic) to 1).
This also solves a long standing issue of these infills producing
anchors for each sweep direction independently, thus possibly
overlapping and overextruding, which was quite detrimental
in narrow areas.
3) Refactored cubic adaptive infill anchroing algorithm
for performance and clarity.
Implementation of 2-opt pairwise exchange iterative improvement
algorithm with an extension to a chain of segments, where
the chain of segments may get flipped during the exchange operation.
The 2-opt exchange algorithm may be quite slow.
