Fix exp perf regression

[slimbuck]

Description

The table introduced in #8030 results in performance regression so use exp instead.

This PR reverts the calc to using build-in exp.

[Copilot]

Pull Request Overview

This PR reverts a performance optimization that used a lookup table for exponential calculations in Gaussian splat rendering back to using built-in exponential functions. The change addresses a performance regression introduced in PR #8030.

• Replaced texture-based exponential lookup table with direct exponential calculations
• Added normExp function to both WGSL and GLSL shader chunks for normalized exponential computation
• Removed exponential table texture creation and management code from the renderer

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/scene/shader-lib/wgsl/chunks/gsplat/frag/gsplat.js	Replaced texture lookup with normExp function for alpha calculation
src/scene/shader-lib/glsl/chunks/gsplat/frag/gsplat.js	Added normExp function and replaced texture lookup with direct calculation
src/scene/renderer/renderer.js	Removed exponential table texture creation, management, and cleanup code

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[Maksims]

Please add to names of PR's a bit more context. "Perf regression" of what?
There is a lot of GS's PR's, would be good to name them with "GS", so it is not confused with other things.