PostProcessingUtils: Add getScreenPosition().

[Mugen87]

Related issue: -

Description

This PR adds getScreenPosition() as a helper to PostProcessingUtils. It also documents the function as well as getViewPosition() so it's more clear what these functions do.

[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	811.49 218.59	811.63 218.63	+137 B +39 B
WebGPU Nodes	811 218.46	811.13 218.5	+137 B +38 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	535.71 144.58	535.71 144.58	+0 B +0 B
WebGPU Nodes	491.83 134.32	491.83 134.32	+0 B +0 B