GLSLNodeBuilder: Fix texelFetch*() code generation with depth textures.

[shotamatsuda]

Fixed #31759.

Description

This PR fixes TextureNode.load() incorrectly returns vec4 when sampling a depth texture on WebGL fallback.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	338.91 79.11	338.91 79.11	+0 B +0 B
WebGPU	578.34 159.33	578.39 159.34	+48 B +19 B
WebGPU Nodes	576.95 159.08	577 159.1	+48 B +19 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	470.78 113.9	470.78 113.9	+0 B +0 B
WebGPU	648.88 175.35	648.93 175.37	+48 B +19 B
WebGPU Nodes	602.97 164.48	603.02 164.5	+48 B +19 B

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[Mugen87]

@shotamatsuda Could you please open a new PR with your changes to fix generateTexture()? I'm afraid the issue became not clear when trying to explain it in #30830 (comment). A new PR would help to launch a new discussion.