Fix GS artifacts

[slimbuck]

Description

This PR fixes a long-standing issue with very bright gaussians showing a hard edge.

Also revert fastExp since it results in banding.

Before:

After:

[Copilot]

Pull Request Overview

This PR fixes rendering artifacts in Gaussian splat rendering by addressing hard edges around very bright gaussians and reverting to native exponential functions to eliminate banding artifacts.

• Removes custom fast exponential approximation that was causing banding
• Implements a normalized alpha calculation to prevent hard edges on bright gaussians
• Applies consistent fixes across both WGSL and GLSL shader implementations

Reviewed Changes

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

File	Description
src/scene/shader-lib/wgsl/chunks/gsplat/frag/gsplat.js	Updates WGSL fragment shader with normalized alpha calculation and removes fast exponential
src/scene/shader-lib/glsl/chunks/gsplat/frag/gsplat.js	Updates GLSL fragment shader with normalized alpha calculation and removes fast exponential

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