Make KD-tree encoder memory linear in the point dimension

[Filyus]

DynamicIntegerPointsKdTreeEncoder allocates memory quadratic in the point dimension, up front, in its constructor. The dimension is the total number of components over all attributes. This change makes the encoder's memory linear in the dimension, the way DynamicIntegerPointsKdTreeDecoder already works since #1241. The encoded bytes are unchanged.

Encoder memory by dimension:

dimension	main, always allocated	this change, at most
3	2.3 KB	1.6 KB
64	1 MB	33 KB
255	17 MB	133 KB
2,048	1.1 GB	1.1 MB
6,375	10 GB	3.3 MB

What this means in practice:

• Three equal points with 2,040 components encode to the same 3,154 bytes with about 1 MB, against 1.07 GB on main.
• Ordinary point clouds encode with about the same work. Clouds of three components take 5% fewer instructions at most levels and 2% more at level 4, where the compiler inlines std::partition differently into the new walk. Clouds with more components take fewer, because the constructor no longer builds 32 × dimension vectors. Callgrind counts of one encode of random 16-bit points, constructor included, built with clang -O3:

level	components	points	main	this change
0	3	50,000	46.1 M	43.6 M
2	3	50,000	50.2 M	47.7 M
4	3	50,000	50.6 M	51.7 M
6	3	50,000	67.6 M	63.7 M
2	6	20,000	25.3 M	24.2 M
4	6	20,000	25.5 M	25.6 M
2	64	2,000	12.7 M	10.7 M

How: the encoder kept a full copy of the node's base and levels for every level of the tree, which is 32 × dimension levels deep. A split changes only one base value and one level. The encoder now keeps a single base and levels vector, records the old values at each split, and restores them when the walk returns to a shallower node. This is the same walk the decoder uses.

Verification:

• 22,000 generated point clouds over all speeds, 1 to 100 components and four point shapes encode to the same bytes on main and on this change. That probe is not part of the change.
• Nine existing PointCloudKdTreeEncodingTest cases fail if the restore is broken, so the existing tests cover it.
• TestKdTreeEncodingOfManyComponents round-trips three equal points with 25 attributes of 255 components, a dimension of 6,375. Equal points are never separated, so the walk goes 11 × 6,375 levels deep, far past the existing tests, which is where the restore does its work. Under AddressSanitizer it peaks at 29 MB with this change and at 12.9 GB on main.
• draco_tests passes, also under AddressSanitizer.