Improve GaussianSplatting for VR

[CedricGuillemet]

Potential improvement for rendering quality and Gaussian Splatting.
See https://forum.babylonjs.com/t/gaussian-splats-rotate-incorrectly-in-vr/50256/41

Current quality:

potential improvement:

I believe it's a small fix related to viewport and focal computation. I could find a fix that worked with a VR rig locally but it doesn't work on real device (no idea why).

Check this as a starting point:

Babylon.js/packages/dev/core/src/Materials/GaussianSplatting/gaussianSplattingMaterial.ts

Line 219 in bf55ce2

let focal = 1000;

[CedricGuillemet]

with https://playground.babylonjs.com/#45KYTJ#44
changing the fov from 1.6 to 2.2 helps having better rendering.
a high value results in smaller splats up to a single point.

[RaananW]

This should also be checked in VR headsets, as the quality there is better than the first image. I will be happy to help with that