A very fast static spatial index for 2D points based on a flat KD-tree, using https://github.com/mourner/kdbush.hpp
Compared to QgsSpatialIndex, this index: - supports single point features only (no multipoints) - is static (features cannot be added or removed from the index after construction) - is much faster! - supports true "distance based" searches, i.e. return all points within a radius from a search point