WebGLRenderer: Implement normal bias fallback.

[Mugen87]

Fixed #21483.

Description

Flat shading with no vertex normals can break shadows on certain devices since shadowWorldNormal might end up with NaN values.

The PR fixes that by introducing a new define that detects whether a geometry has vertex normals or not and only then computes shadowWorldNormal. Otherwise it uses the fallback value.

In WebGPURenderer, we compute the normal bias in the fragment shader so the computation operates on valid normals. Hence, the test asset from #21483 works fine with WebGPURenderer for both WebGL and WebGPU. However, doing the same in WebGLRenderer requires more refactoring and the presented approach is a sensible fallback for this edge case (not every user enabling shadows makes use of normalBias).

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	361.21 85.78	361.4 85.82	+191 B +45 B
WebGPU	637.46 177.05	637.46 177.05	+0 B +0 B
WebGPU Nodes	635.58 176.76	635.58 176.76	+0 B +0 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	493.71 120.45	493.9 120.5	+191 B +45 B
WebGPU	709.2 191.87	709.2 191.87	+0 B +0 B
WebGPU Nodes	658.42 179.16	658.42 179.16	+0 B +0 B

[Mugen87]

The PR actually fixes the broken self-shading of the animated horses in webgl_shadowmap_performance.