PMREMGenerator: Support reversed depth

[WestLangley]

As suggested in #31413 (comment).

Fixes #31413.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	338.12 78.86	338.22 78.88	+99 B +29 B
WebGPU	560.22 155.1	560.22 155.1	+0 B +0 B
WebGPU Nodes	559.14 154.89	559.14 154.89	+0 B +0 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	469.39 113.53	469.49 113.56	+99 B +33 B
WebGPU	635.38 172.03	635.38 172.03	+0 B +0 B
WebGPU Nodes	590.03 161.27	590.03 161.27	+0 B +0 B