VR Passthrough Support

Blender could use support overlaying the 3D content onto the real world when using the VR mode.

The OpenXR specification has built-in passthrough support with the environment blend modes. For basic passthrough support, Blender would have to set the blend mode to Alpha-Blend, and submit an alpha cutout in the color buffers. Blender already supports masking out transparent backgrounds, this could be enabled by the user with the composition system.

Unfortunately not many OpenXR runtimes support the core OpenXR passthrough system, instead opting to use proprietary extensions. AFAIK only the Varjo runtime currently does. I've added unofficial support for the SteamVR runtime as well. github.com/Rectus/openxr-steamvr-passthrough