PMREM: Simplify GGX VNDF importance sampling.

[Mugen87]

Related issue: #32692

Description

The PR simplifies importanceSampleGGX_VNDF() for both renderers since the second parameter V always has the value of (0, 0, 1). That makes certain computations redundant. By removing them, we also fix one of the DirectX warnings reported in #32692.

warning X4008: floating point division by zero

I've tested the fix on a Windows system and the warning is indeed gone.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	355.41 84.51	355.15 84.44	-253 B -69 B
WebGPU	621.21 172.51	621.09 172.46	-118 B -49 B
WebGPU Nodes	619.82 172.27	619.7 172.22	-118 B -51 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	487.36 119.29	487.11 119.23	-253 B -64 B
WebGPU	691.86 187.79	691.75 187.75	-118 B -46 B
WebGPU Nodes	641.66 174.96	641.55 174.91	-118 B -45 B