Fixes and improvements for Polygon.removeDuplicateAxes()

Spend less cycles to find duplicate axes (and don't delete useful ones). The process has also been optimized: f.e. previously using a 16 sides regular polygon incorrectly generated and made use of 16 axes, now that's reduced to 8 (as it should be, noting that in a regular shape each couple of opposing edges has the same orientation).

See #291 and #292.