WebGPURenderer: PMREMGenerator - calculate blur weights and parameters once

[aardgoose]

Address TODO and uses userData() instead of material uniforms for per pass constants.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	339.37 79.06	339.37 79.06	+0 B +0 B
WebGPU	478.22 132.63	478.65 132.72	+430 B +90 B
WebGPU Nodes	477.68 132.51	478.11 132.6	+430 B +89 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	464.82 112.02	464.82 112.02	+0 B +0 B
WebGPU	546.95 148.2	547.64 148.38	+687 B +179 B
WebGPU Nodes	502.83 137.91	503.52 138.1	+687 B +194 B

[mrdoob]

/ping @elalish

[elalish]

I guess it's fine if it creates the same pixels. I didn't write that TODO, so I'm not sure what this refactor is for. I generally hope for a refactor to result in less code rather than more. If other folks think this is more clear, then go for it.