TSL: Fix getBitangent().

[Mugen87]

Related issue: #31045 (comment)

Description

The new asset for the tone mapping demos exhibits an issue in WebGPURenderer. Turns out, #30837 introduced a minor regression in getBitangent(). The returned varying referred to a wrong value.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	336.29 78.32	336.29 78.32	+0 B +0 B
WebGPU	548.2 152.02	548.2 152.02	+0 B +0 B
WebGPU Nodes	547.55 151.86	547.55 151.86	+0 B +0 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	466.23 112.58	466.23 112.58	+0 B +0 B
WebGPU	622.98 168.71	622.98 168.71	+0 B +3 B
WebGPU Nodes	577.85 158	577.85 158	+0 B +1 B

[sunag]

Thank you! I'm working on a better solution for normals to solve the flatShading bug and remove the transformed prefix, this involves some TSL improvements to make the solution simpler.

/cc @WestLangley @mrdoob