quadtree: sort KNearest results closest first

[paulmach]

as requested here https://github.com/paulmach/orb/issues/67

The results of the quadtree.KNearest were just the "heap" order. Now they are sorted closest first.

It is slower however, would probably need to implement a custom heap to get rid of the allocs. I did that for the visvalingam simplification here. Hopefully generics will allow this to be simple and performant.

benchmark                         old ns/op     new ns/op     delta
BenchmarkRandomKNearest100-12     31442         43139         +37.20%

benchmark                         old allocs     new allocs     delta
BenchmarkRandomKNearest100-12     269            369            +37.17%

benchmark                         old bytes     new bytes     delta
BenchmarkRandomKNearest100-12     12083         15280         +26.46%

[paulmach]

I implement an "internal" maxHeap vs using the std lib one. This makes it twice as fast AND returns sorted results. Allocations are now ~K vs. "nodes seen".

If you were doing this 1000s of times for the same quadtree you could implement something to reuse the maxHeap memory. Some sort of KNearestContainer and create one per goroutine/thread. That would put you down to 2-4 allocations per KNearest search.

benchmark                         old ns/op     new ns/op     delta
BenchmarkRandomKNearest10-12      3650          2094          -42.63%
BenchmarkRandomKNearest100-12     32616         15957         -51.08%

benchmark                         old allocs     new allocs     delta
BenchmarkRandomKNearest10-12      34             14             -58.82%
BenchmarkRandomKNearest100-12     269            104            -61.34%

benchmark                         old bytes     new bytes     delta
BenchmarkRandomKNearest10-12      1457          472           -67.60%
BenchmarkRandomKNearest100-12     12081         3432          -71.59%