Use native OpenXR instead of OpenVR addon

[kb173]

Godot 4.0 natively supports VR: https://docs.godotengine.org/en/latest/tutorials/xr/setting_up_xr.html

[kb173]

Some work for this was done in 97fd6a0b. The VR perspective now works. However, there are some issues with the current state of OpenXR in Godot 4:

• Complex shaders seem to be problematic, e.g. the cloud shader with default settings produces a crash spamming this output, but with reduced complexity it does work:

  OpenXR: Failed to locate view space in play space [ XR_ERROR_TIME_INVALID ]
  OpenXR: failed to locate space! [ XR_ERROR_TIME_INVALID ]

• Some weird behavior, e.g. 11beac8a was required to get the game to start at all
• High VRAM usage (needs testing on the more powerful PC)

[kb173]

Luckily I was mostly wrong! Everything works with the newly updated FirstPersonVR node, it's just that SteamVR's automatic resolution adaption does not seem to do its job well at all. When manually setting the resolution to 100%, the LandscapeLab runs fine with VR, even the sky works with no changes. I assume that the automatic setting brought the resolution up way too high for some reason, causing VRAM issues.

Since the FirstPersonVR node is functional and can be added as needed, this issue is finished.