fastExp implementation for gsplat rendering

[slimbuck]

This PR implements a fast approximation for the exp function and makes use of it in the gsplat shader for both WebGL and WebGPU.

Resulting images are unchanged, but we require more performance testing to determine the speed gain up on various devices. My macbook shows only a small speedup (< 1%), but the hope is larger speedups are found on lower end devices.

[Copilot]

Pull Request Overview

This PR adds a faster approximation of the exponential function (fastExp) and replaces the standard exp call in the gsplat fragment shaders for both WGSL and GLSL to improve rendering performance.

• Introduces fastExp function with constants and bit-cast logic in WGSL and GLSL shader chunks.
• Replaces exp(-A * 4.0) with fastExp(-A * 4.0) for alpha evaluation in both shader variants.

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	Added fastExp implementation and replaced exp call.
src/scene/shader-lib/glsl/chunks/gsplat/frag/gsplat.js	Added fastExp implementation and replaced exp call.

Comments suppressed due to low confidence (2)

src/scene/shader-lib/wgsl/chunks/gsplat/frag/gsplat.js:22

• [nitpick] Consider documenting the valid input range and expected approximation error of fastExp, and briefly explain how the bias constant is derived.

// Fast approximate e^x based on https://nic.schraudolph.org/pubs/Schraudolph99.pdf

src/scene/shader-lib/wgsl/chunks/gsplat/frag/gsplat.js:25

• Add unit tests or shader benchmarks to validate fastExp accuracy against the standard exp over the intended input range.

fn fastExp(x: f32) -> f32 {