TSL: Fix positionWorld used in material.positionNode and fragment-stage

[sunag]

Description

positionWorld was performant once if used on positionNode and colorNode at the same time. However positionNode should make changes to positionWorld if used in fragment-stage, this PR re-assigns positionWorld if used in both.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	339.21 79.01	339.21 79.01	+0 B +0 B
WebGPU	485.91 134.87	486.25 134.97	+340 B +98 B
WebGPU Nodes	485.38 134.77	485.72 134.86	+340 B +93 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	465.1 112.06	465.1 112.06	+0 B +0 B
WebGPU	555.02 150.3	555.36 150.39	+340 B +84 B
WebGPU Nodes	510.87 140.01	511.21 140.1	+340 B +86 B