Add fog support for Gaussian splat rendering

[mvaligursky]

Adds scene fog to both GSplat rendering paths (quad renderer and compute rasterizer), applied per-splat in linear color space.

Changes:

• Add fog to GSplat quad renderer vertex shader via prepareOutputFromGamma, which now accepts a depth parameter and applies fog in linear space between decode and tonemapping
• Add per-splat fog to compute GSplat rasterizer during the projCache-to-shared-memory load, with proper gamma decode/encode
• Create shared fogMathPS WGSL chunk with pure math functions (evaluateFogFactorLinear, evaluateFogFactorExp, evaluateFogFactorExp2) reusable by both fragment/vertex and compute shaders
• Refactor WGSL fogPS chunk to use fogMathPS and support vertex shader usage via VERTEXSHADER conditional
• Resolve fog parameters from camera override (camera.fogParams) or scene default (scene.fog) in GSplat manager
• Add fog type as an additive shader variant key in the compute dispatch set
• Add GSplatParams.useFog property to allow disabling fog on splats even when the scene has fog enabled

API Changes:

• GSplatParams.useFog (boolean, default true): When set to false, Gaussian splats ignore scene/camera fog settings

Examples:

• Weather example: add fog density/exposure sliders, "Splat Fog" toggle, exposure and fog per preset (None/Snow/Rain), fix preset property application
• LOD streaming example: add fog density slider

Performance:

• No overhead when fog is disabled — shader variants without fog remain unchanged
• Fog uniforms only added to compute uniform buffer when fog is active