Small optimization on an incredibly hot codepath.
Inline comparable_distance_between_points

It was called on an incredibly hot codepath from a single place. At 12313276 calls
on my test .stl, the sub call overhead alone was a significant perf hit.
Only look up $point's X and Y once, rather than once on every pass through the loop.
(Those lookups are expensive)
no functional change. only return from one place for clarity
split apart the math in nearest_point_index and short-circuit if we know the candidate is no good
Merge pull request #1107 from obra/nearest_point_index_perf_part_2

Nearest point index perf work part 2