Use exp table for gs

[slimbuck]

Followup to #8030.

Description

Instead of using expensive exp call per pixel when rendering gaussians, generate an exp table and sample that instead. Also fold in the normalization factor.

[Copilot]

Pull Request Overview

This PR optimizes Gaussian splat rendering performance by replacing expensive per-pixel exponential function calls with a precomputed lookup table. The optimization precomputes the exponential values and normalization factors into a texture that can be sampled efficiently during rendering.

• Replaces costly exp(-A * 4.0) computations with texture lookups in both GLSL and WGSL shader fragments
• Creates a precomputed exponential lookup table texture with normalization factors baked in
• Integrates the lookup table into the renderer's uniform system for automatic binding

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/scene/shader-lib/wgsl/chunks/gsplat/frag/gsplat.js	Replaces exponential calculation with texture sampling using expTable
src/scene/shader-lib/glsl/chunks/gsplat/frag/gsplat.js	Replaces exponential calculation with texture sampling using expTable
src/scene/renderer/renderer.js	Adds exponential lookup table creation and management infrastructure

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