feat: linear lighting

[jiayev]

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• Linear Lighting is a core feature making most inner gamma conversions controllable. Make most shaders output linear results and do gamma correction after tonemapping.
• Add multiple gamma and multiplier sliders, to control color, effects, sky, etc. separately.
• I'm not sure this is fully ready for PR. But it's important and should be adapted to most features, including new features. Getting this merged would help its development.
• Future plan: adding flags for color or lights, supporting real linear inputs.
• Not for merging at the moment. I'd like to check some ai review.

Summary by CodeRabbit

• New Features

  • Added a configurable Linear Lighting system with UI controls, VR support, and per-frame runtime toggles.

• Improvements

  • Replaced many gamma-paths with irradiance/radiance-aware conversions for more consistent ambient, diffuse, specular, fog, sky, water, subsurface and volumetric lighting.
  • Introduced centralized color utilities and multipliers to standardize lighting/ effect scaling.

• Bug Fixes

  • Moved fade/dimming logic to dedicated fade parameters to avoid unintended color scaling.

• Chores

  • Added shared linear-lighting settings and registered the feature for per-frame buffers and global feature lists.