TSL: Add exponentialHeightFogFactor().

[Mugen87]

Related issue: #32613 (comment)

Description

I've experimented with fog code lately and this is the simplest exponential height fog I can think of. You have a density parameter to control the global fog density (similar to densityFogFactor()) and an additional height parameter in world space that defines the height (y-coordinate) threshold when fragments are affected by fog.

Link for testing: https://rawcdn.githack.com/Mugen87/three.js/c0a7e5ee23e4ec5d5ef209727ed352e97aa192f1/examples/webgpu_fog_height.html

I'll temporarily update the builds for testing.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	355.36 84.47	355.36 84.47	+0 B +0 B
WebGPU	621.34 172.52	621.49 172.57	+158 B +50 B
WebGPU Nodes	619.94 172.27	620.1 172.32	+158 B +51 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	487.23 119.23	487.23 119.23	+0 B +0 B
WebGPU	692.28 187.78	692.41 187.81	+131 B +33 B
WebGPU Nodes	642.08 175	642.21 175.03	+131 B +34 B