WGSLNodeBuilder: Fix varying indexes

[sunag]

Fixes #33222

Description

This fixes the indices of the varyings since the optimization system may not find a varying interpolation necessary and replace it with regular variables.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	360.82 85.68	360.82 85.68	+0 B +0 B
WebGPU	632.75 175.53	632.76 175.53	+12 B -4 B
WebGPU Nodes	630.87 175.24	630.88 175.24	+12 B -5 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	493.02 120.25	493.02 120.25	+0 B +0 B
WebGPU	705.18 190.51	705.19 190.5	+12 B -4 B
WebGPU Nodes	654.4 177.79	654.41 177.78	+12 B -5 B