fixed KD tree neighbour search, finished refactoring of raycasts,
substantially improved performance of seam placement - seams
are now precomputed and their indexes stored such that place_seam does almost nothing.
producing better shortest path estimate than the "closest next neighbor"
heuristics. The new greedy algorithm utilizes KD tree for closest
end point search, and builds a graph to detect loops.
PerimeterGenerator newly uses the optimized TSP algorithm.
ExtrusionEntity has been refactored / simplified.
