NodeMaterial: Make sure fog is applied only once.

[Mugen87]

Fixes #31462

Description

Since fog() relies on output and there are assignments to output in NodeMaterial.setup(), there is a side-effect that leads to #31462. Using toStack() avoids the duplication.

@sunag There might be a better point for calling toSack() or even another solution but the PR in its current form at least fixes #31462.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	338.04 78.86	338.04 78.86	+0 B +0 B
WebGPU	559.09 154.77	559.1 154.77	+10 B +2 B
WebGPU Nodes	558.01 154.55	558.02 154.56	+10 B +3 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	469.29 113.56	469.29 113.56	+0 B +0 B
WebGPU	634.38 171.81	634.39 171.81	+10 B +6 B
WebGPU Nodes	589.51 161.12	589.52 161.12	+10 B +2 B

[sunag]

It's all good to me 👍