TSL: Move TriplanarTexturesNode to triplanarTextures() Fn

[sunag]

Description

Simplification. Move TriplanarTexturesNode to triplanarTextures() Fn

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	337.54 78.73	337.54 78.73	+0 B +0 B
WebGPU	556.43 154.15	556.06 154.04	-368 B -108 B
WebGPU Nodes	555.78 153.99	555.41 153.88	-368 B -109 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	468.74 113.38	468.74 113.38	+0 B +0 B
WebGPU	632.24 171.08	632.24 171.08	+0 B +0 B
WebGPU Nodes	587.24 160.39	587.24 160.39	+0 B +0 B