WebGPURenderer: Unify uniformGroup bindings across shader stages

[sunag]

Related issue: #32534

Description

Fixes duplicate binding declarations for uniformGroups when the same group is used in both vertex and fragment shaders. Previously, each shader stage would increment the binding index independently, causing mismatched binding indices when shared.

No changes were needed for GLSLNodeBuilder.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	355.34 84.47	355.34 84.47	+0 B +0 B
WebGPU	618.69 171.84	618.86 171.89	+174 B +49 B
WebGPU Nodes	617.29 171.6	617.47 171.65	+174 B +49 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	487.5 119.33	487.5 119.33	+0 B +0 B
WebGPU	690.95 187.49	691.12 187.54	+174 B +46 B
WebGPU Nodes	640.79 174.72	640.96 174.77	+174 B +47 B