GLSLNodeBuilder: Fix varying color type

[sunag]

Description

Fix type conversion if used varying in WebGL Backend.

material.colorNode = uniform( color( 0x00ff00 ) ).varying();

/cc @brunosimon

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	339.13 79	339.13 79	+0 B +0 B
WebGPU	482.81 133.76	482.84 133.77	+28 B +8 B
WebGPU Nodes	482.27 133.66	482.3 133.67	+28 B +9 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	464.59 111.96	464.59 111.96	+0 B +0 B
WebGPU	550.87 149.23	550.9 149.24	+28 B +8 B
WebGPU Nodes	506.75 138.95	506.78 138.96	+28 B +8 B