ChainMap: Avoid a redundant internal weak map.

[ycw]

Description

Issue: ChainMap holds value in a redundant weakmap, see https://jsfiddle.net/rebw4hvn/:

chainMap.set([key0, key1], value)

// expected
chainMap.weakMap.get(key0).get(key1) === value

// actual
chainMap.weakMap.get(key0).get(key1).get(key1) === value

This PR fixed that by using just enough weakmaps.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	339.49 79.09	339.49 79.09	+0 B +0 B
WebGPU	489.82 135.88	489.83 135.88	+6 B +5 B
WebGPU Nodes	489.29 135.76	489.29 135.77	+6 B +6 B

🌳 Bundle size after tree-shaking

Minimal build including a renderer, camera, empty scene, and dependencies.

	Before	After	Diff
WebGL	465.35 112.14	465.35 112.14	+0 B +0 B
WebGPU	559.3 151.37	559.31 151.37	+6 B +1 B
WebGPU Nodes	515.38 141.15	515.38 141.15	+6 B -1 B