WebGLShadowMap: Move point light shadow logic from PointLightShadow.

[mrdoob]

Description

Moves _cubeDirections, _cubeUps, and the point light shadow camera setup logic from PointLightShadow to WebGLShadowMap.

This aligns with how PointShadowNode handles point light shadows for the WebGPU renderer, where the cube face directions and camera setup are managed by the shadow map renderer rather than the shadow class itself.

PointLightShadow is now simplified to just the constructor and type flag.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	350.26 83.01	350.9 83.23	+643 B +222 B
WebGPU	616.03 171	616.03 171	+0 B +0 B
WebGPU Nodes	614.64 170.74	614.64 170.74	+0 B +0 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	482.3 117.81	482.96 118.03	+658 B +220 B
WebGPU	687.42 186.76	686.7 186.56	-725 B -205 B
WebGPU Nodes	637.26 173.94	636.54 173.77	-725 B -175 B