WIP: OpenXR integration

[blaind]

One take at issue #3219

PR checklist:

• [ ] make succeeds (on *nix)
• [ ] make reftests succeeds
• [ ] tested examples with the following backends:

I've made a proof-of-concept for OpenXR rendering on Bevy engine, using wgpu & underlying stack. See https://github.com/blaind/xrbevy

The code in this pull request is not even close to merge-able, but I thought its good idea to open it for further discussion.

For architecture of the pull request, see https://github.com/blaind/xrbevy/blob/main/docs/architecture.md (evolving document)

See also https://github.com/gfx-rs/wgpu-rs/pull/910 and https://github.com/gfx-rs/wgpu/pull/1387

[blaind]

Even though this renders, there might be some parts from OpenXR api that are missing currently:

• https://www.khronos.org/registry/OpenXR/specs/1.0/html/xrspec.html#xrCreateVulkanInstanceKHR creation of vulkan device through openxr API
• https://www.khronos.org/registry/OpenXR/specs/1.0/html/xrspec.html#_swapchain_interactions specific swapchain image type?

[blaind]

I also drafted a diagram about openxr entities:

I wonder if the swapchain handling could be integrated into gfx (or wgpu?)

[blaind]

I'll close this - there's a better encapsulation approach with raw handles that's being continued at https://github.com/gfx-rs/wgpu/pull/1415 && https://github.com/gfx-rs/wgpu/pull/1609