WebGPURenderer: Fix point light shadows.

[mrdoob]

Related issue: #32303

Description

The WebGL shadow modernization PR removed the 2D texture atlas configuration (_frameExtents, _viewports, _cubeDirections, _cubeUps) from PointLightShadow since WebGL now uses native cube depth textures. However, PointShadowNode still relies on the 2D atlas approach for WebGPU. This PR adds the necessary viewport and cube face configuration directly to PointShadowNode, making it self-contained and independent of PointLightShadow's internal implementation.

I plan to use native cube depth textures in WebGPURenderer too but fixing this for now.

(Made using Claude Code with Opus 4.5)

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	350.29 83.05	350.29 83.05	+0 B +0 B
WebGPU	612.26 170.09	612.89 170.29	+630 B +198 B
WebGPU Nodes	610.86 169.82	611.49 170.01	+630 B +188 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	482.23 117.82	482.23 117.82	+0 B +0 B
WebGPU	683.13 185.79	683.78 186	+652 B +204 B
WebGPU Nodes	632.96 172.99	633.61 173.18	+652 B +193 B