TSL: Minor fixes.

[Mugen87]

Related issue: -

Description

While working on a new post processing node, I have noticed some minor issues:

• luminance() returns a float, not a vec3.
• ~ is a unary prefix operator. It comes before the single variable (operand) it modifies which means the TSL function bitNot() must have a parameter length of 1.
• Some algorithms require HALF_PI as a constant. The PR adds it as a new TSL constant.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	338.91 79.11	338.91 79.11	+0 B +0 B
WebGPU	579.78 159.79	579.84 159.82	+56 B +30 B
WebGPU Nodes	578.39 159.55	578.44 159.58	+56 B +27 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	470.77 113.9	470.77 113.9	+0 B +0 B
WebGPU	649.42 175.56	649.42 175.56	+0 B +2 B
WebGPU Nodes	603.52 164.72	603.52 164.72	+0 B +3 B