WebGPURenderer: webgl-fallback - fix uniform/texture index allocation

[aardgoose]

Related issue: #29610

CSMshaderNode exposes a faulty assumption in allocation of uniform buffer indices.
Correct Index allocation to take account of uniform buffers of arrays.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	689.92 170.93	689.92 170.93	+0 B +0 B
WebGPU	814.71 219.35	814.82 219.39	+108 B +39 B
WebGPU Nodes	814.22 219.22	814.33 219.26	+108 B +40 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	462.94 111.81	462.94 111.81	+0 B +0 B
WebGPU	537.56 145.04	537.67 145.07	+108 B +38 B
WebGPU Nodes	493.67 134.77	493.78 134.81	+108 B +40 B