OpenXR support for GaussianViewer

• Add a new SIBR rendering mode OpenXRRdrMode to support Headed-Mounted dislay (HMD) OpenXR devices
• GaussianViewer can use this rendering mode with --rendering-mode 2 option to render 3D Gaussian Splatting scene to two-views headset display through OpenXR stack.
• Works on Windows and Linux (through the SteamVR OpenXR runtime)

Note: OpenXRRdrMode does not support actions (aka controller buttons). But two VR experiences are available:

• Free world standing: only headset's displacement allows move within the 3DGS scene
• Seated: controls with remote keyboard or mouse are available in addition to headset's displacement

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How to test:

Windows (Meta Quest 1/2/3/Pro):

1. Install Desktop PC Oculus Application: https://www.meta.com/en-gb/help/quest/articles/headsets-and-accessories/oculus-rift-s/install-app-for-link/
2. Setup Quest Link: https://www.meta.com/en-gb/help/quest/articles/headsets-and-accessories/oculus-link/set-up-link/
3. Headset should be in Oculus AirLink Home screen (white background)
4. Run gaussianViewer -m <dataset_path> --rendering-mode 2

5. You can try Free world standing and Seated VR experiences
6. If you are experiencing lags in the headset, try to lower the rendering resolution by changing Down scale factor slider value.

Linux (through Steam):

1. Install Steam and SteamVR
2. Make SteamVR the OpenXR default runtime (Settings > OpenXR > SET STEAMVR AS OPENXR RUNTIME)
3. Restart SteamVR
4. Run gaussianViewer -m <dataset_path> --rendering-mode 2

I tested it with an HTC Vive Pro with beta - SteamVR Beta Update on Ubuntu distribution.