VR support

[ninjamode]

Hi,

this adds VR support as mentioned in #17. It is based on the work in https://github.com/ninjamode/Unity-VR-Gaussian-Splatting

Changes are:

• Replace P and VP matrice usage with Unity builtins
• Update DeviceRadixSort to an newer version that has better GPU support (needed for Quest 3), see Different Android devices produce different results. b0nes164/GPUSorting#4
• Uses the eye texture size in screen size calculations when available

Not ported over is the sorting for eye center. It's quite helpful, but not implemented nice enough for me to want to port it over, so maybe should be mentioned in the readme as a optimization that's maybe worth it? Not strictly needed, especially with Desktop VR. Sample implementation can be found here: https://github.com/ninjamode/Unity-VR-Gaussian-Splatting/blob/main/package/Runtime/GaussianSplatRenderer.cs

Anyway, this does not add any Readme changes, but you might want to adjust the VR notice.

Tested and works on: Desktop Steam VR (HTC Vive), Varjo Devices (tested on Aero), Quest 3 and Quest Pro via Link and standalone. Probably also works on other devices, such as mobile phones? It's to keep in mind that GS rendering uses quite some resources, so keep the Gaussian count low on mobile devices, try not to be super close (fillrate issue?), don't sort every frame.

How To:

1. Use URP (Standard Pipeline has more matrix issues. Can probably be fixed, I didn't see the point)
2. Enable XR plugin Management
3. Enable and start OpenXR on init
4. Set Render Mode to Multi-Pass

Thats it.
If you ever get the chance to try it you should, it's quite impressive. If you feel like it, I'd be happy if you could mention my repository and/or the fact that this was done as a part of a research project that can be cited (see https://github.com/ninjamode/Unity-VR-Gaussian-Splatting/?tab=readme-ov-file#related ).

In any case, thank you for this project, and let me know If you'd like to see any changed. Maybe someone with a VR headset can test this and confirm I didn't accidentally break anything.

[ninjamode]

Probably worth mentioning that this does NOT add support for Apple Vision Pro, due to not supporting Single Pass Instanced Rendering.

[aras-p]

This is excellent, thank you! I'll do some non-VR testing here locally and will merge / update the readme soon.