TSL: Avoid vector reuse in luminance() input

[donmccurdy]

Since the luminance() function could be called multiple times with different coefficients, and I'm not sure whether the vector is stored internally by vec3(vector), perhaps it's better not to reuse a local variable here.

/cc @sunag does this sound correct?

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	688.13 170.5	688.13 170.5	+0 B +0 B
WebGPU	807.26 217.48	807.25 217.46	-9 B -18 B
WebGPU Nodes	806.77 217.34	806.76 217.32	-9 B -19 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	462.85 111.78	462.85 111.78	+0 B +0 B
WebGPU	536.06 144.73	536.06 144.73	+0 B +0 B
WebGPU Nodes	492.14 134.46	492.14 134.46	+0 B +0 B

[sunag]

Yes, sounds correct to me too. I think we can remove the spread syntax (...) too.