PMREMGenerator: Improve GGX VNDF accuracy and match Blender roughness

[mrdoob]

Description

This PR improves the accuracy of the GGX VNDF filtering in PMREMGenerator to better match standard PBR implementations like Blender by tuning the blurStrength scaling factor to 0.0 + targetRoughness * 1.25.

Reasoning

We sample from the previous mip level to maintain high performance (~10ms generation time). However, this introduces implicit blur from the downsampling and bilinear interpolation. The adjusted blurStrength factor compensates for this by reducing the applied roughness at lower LODs, preventing the "over-blurring" issue at low roughness values while ensuring the higher roughness levels converge correctly.

Before	After	Blender

The middle column is still too crisp but overall is much better.

Note: Need to find a easier way to make renders with the same camera than the three.js scene 😅

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	356.18 86.47	356.18 86.47	-1 B -2 B
WebGPU	616.43 173.08	616.42 173.08	-1 B -1 B
WebGPU Nodes	615.03 172.83	615.03 172.83	-1 B -1 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	488.05 121.29	488.05 121.29	-1 B -1 B
WebGPU	688.66 188.99	688.66 188.99	-1 B -1 B
WebGPU Nodes	630.38 172.22	630.38 172.22	-1 B +0 B