OpenXR: Virtual Reality Rendering

Hi! This project sounds awesome. I just discovered it and wanted to talk about XR support.

One could assume that this could be implemented as an external module. However, the OpenXR library creates one or more render targets (for Vulkan/GL/...) based on the physical XR hardware. Therefore, the XR module needs to talk to the rendering module. As a consequence, a few generic changes to the rendering module may possibly be required, changes that are most probably unspecific to OpenXR. That's why it's reasonable to talk about this now already.

I'd like to look into OpenXR, which already has Rust bindings. However, the architecture should probably be prototyped in collaboration, as I don't know all the bevy stuff yet. I'll read all the bevy documentation and then come back to this issue.

I'd be happy to hear your thoughts to this! Cheers