WebGPURenderer: Fix mat2() arguments

[RenaudRohlinger]

Description
This PR fixes mat2() incorrectly interpreted as vec4 in the Node Builder. This fix ensures correct type handling.

This contribution is funded by Utsubo

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	336.36 78.33	336.36 78.33	+0 B +0 B
WebGPU	518.85 144	518.88 144.01	+26 B +13 B
WebGPU Nodes	518.32 143.88	518.34 143.9	+26 B +13 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	465.35 112.17	465.35 112.17	+0 B +0 B
WebGPU	590.68 159.95	590.71 159.97	+26 B +14 B
WebGPU Nodes	545.99 149.43	546.01 149.44	+26 B +13 B

[RenaudRohlinger]

In WGSL specs, matrices without arguments are Zero Valued. Should we align to the specs or keep identity (GLSL behavior) like now?
https://www.w3.org/TR/WGSL/#zero-value-builtin-function

/cc @Mugen87 @sunag

[sunag]

I think it's better to follow GLSL behavior as we are doing with other things in order to facilitate TSL Transpiler and code migration.