kdtree: Use arena allocation for nodes.

Slightly faster overall for both tree-building and matching.
kdtree: Remove all support for reverse searches.

Provided little-to-no quality benefit at a 2x speed cost.
As a bonus, matching is faster now due to less complicated checks.
kdtree: Remove mapping from timings.
kdtree: Properly write points to list in uniqueness search.

Quality improves for animated and HD scenes, basically anything
with large amounts of uniform blocks.