Performance: get_surface_distance should use a KDTree for euclidian distance on CPU #8909

Currently it does a full euclidian distance transform which computes all the distances between all of the points. On a GPU this is an embarrassingly parallel computation, but on a CPU we can get significant speedups from using a KDTree to just compute the distances to the nearest edge voxel.

I'll raise a PR - just creating this issue for tracking purposes.