GTAONode: Fix AO with WebGL backend.

[Mugen87]

Related issue: #29588

Description

Second attempt to fix the AO in webgpu_postprocessing_ao when using a WebGL backend.

The issue is indeed different NDC Z conventions in WebGL and WebGPU. There is now a core function getViewPosition() that takes care of that. The previous PR failed because getSceneUvAndDepth() is actually compatible for both WebGL and WebGPU.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	689.82 170.9	689.82 170.9	+0 B +0 B
WebGPU	809.73 218.11	809.96 218.19	+232 B +74 B
WebGPU Nodes	809.24 217.98	809.47 218.05	+232 B +73 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	462.84 111.78	462.84 111.78	+0 B +0 B
WebGPU	535.76 144.59	535.76 144.59	+0 B +0 B
WebGPU Nodes	491.87 134.33	491.87 134.33	+0 B +0 B