PMREMGenerator: Replace separable blur with spiral blur and optimize vertex shader

[mrdoob]

Description

This PR replaces the previous separable blur (latitudinal/longitudinal) in PMREMGenerator with a more efficient spiral blur kernel, and optimizes the internal vertex shader.

Changes

• Spiral Blur: Implemented a Golden Angle spiral blur shader to provide a better distribution of samples.
• Split Sigma: Updated _blur to perform two passes with split sigma (sigma / sqrt(2)), effectively doubling the sample density and quality while maintaining high performance.
• Shader Optimization: Precomputed the outputDirection attribute in JavaScript, removing the faceIndex attribute and the branching getDirection logic from the vertex shader.
• Cleanup: Removed unused poleAxis parameter, sigmas array, EXTRA_LOD_SIGMA constant, and unnecessary object allocations.

(Made using https://jules.google/ and https://antigravity.google/)

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	350.29 83.05	350.18 82.92	-110 B -129 B
WebGPU	612.89 170.29	612.89 170.29	+0 B +0 B
WebGPU Nodes	611.49 170.01	611.49 170.01	+0 B +0 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	482.23 117.82	482.12 117.7	-112 B -127 B
WebGPU	683.78 186	683.78 186	+0 B +0 B
WebGPU Nodes	633.61 173.18	633.61 173.18	+0 B +0 B