Integrating VR into simulate

[nishbo]

The current branch uses OpenVR (very close to the original mjvive project) to render the scenes in VR. It is turned on via MUJOCO_BUILD_SIMULATE_VR cmake option. If the VR could not be initialized, the simulate runs normally. I aimed at minimizing the number of changes/additions to simulate.cc. Supports definition of VR transformation in the model (instead of hardcoded default) - see balloons model.

Tested on Windows 10/11, using Steam VR:
Valve Index - works
HTC Vive XR Elite - works, streaming using Vive Streaming Hub
Meta Quest 3 - technically works (through Steam Link), but the projections are wrong, will need fixing.

Current problems:

1. Not enough documentation. Suggestions on where to add docs are welcome.
2. I need to manually add openvr_api.lib to the linker include directories in Visual Studio and copy openvr.dll into the output directory. Probably can be fixed at the cmake level, not sure how, suggestions welcome.
3. Local copy of glad.

Directions before merging:

1. Switching between VR transformations (partially complete).
2. Fixing Quest 3.
3. Adding UI bars to VR.
4. Adding controllers.

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[nimrod-gileadi]

This isn't auto-generated, right? Could this be taken from the checked out repository, instead of checking in our own copy?