WebGLRenderer: Fix pointlight shadows with reversed depth buffer.

[Mugen87]

Related issue: #31661

Description

The PR fixes the viewZ to perspective depth computation for point light shadows with Basic and PCF shadow maps when reversed depth buffer is used.

It would also be possible to just add dp = 1.0 - dp; but that would be an additional computation that we can save by using an updated formula for reversed depth buffer in the first place.

Note: This is something we can later hide in the viewZToPerspectiveDepth() TSL function.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	355.15 84.44	355.57 84.48	+416 B +45 B
WebGPU	621.09 172.46	621.09 172.46	+0 B +0 B
WebGPU Nodes	619.7 172.22	619.7 172.22	+0 B +0 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	487.39 119.33	487.81 119.38	+416 B +42 B
WebGPU	692.03 187.85	692.03 187.85	+0 B +0 B
WebGPU Nodes	641.83 175.02	641.83 175.02	+0 B +0 B

[Mugen87]

FYI: In this BasicShadowMap version, we could also keep the "viewZ to perspective depth" formula as it is and just do this instead:

float depth = textureCube( shadowMap, bd3D ).r;

#ifdef USE_REVERSED_DEPTH_BUFFER
    depth = 1.0 - depth;
#endif

shadow = step( dp, depth );

However, that does not work for PCF where we don't directly sample the shadow map but use a compare operation.