The Blender's VkInstance cannot be shared with OpenXR VkInstance. The
reason is a chicken and egg problem where OpenXR needs to be started
before Vulkan. OpenXR can add special vulkan specific requirements
(instance&device) that are only available when the user starts an OpenXR
session.
The goal implementation is to share memory between both instances using
[VK_KHR_external_memory](https://registry.khronos.org/vulkan/specs/latest/man/html/VK_KHR_external_memory.html) and related extensions. However this seems
to be a bridge to far as a initial step. Reason: There are not that many
samples/ guides and documentation to be found to handle the workflow that
we require. We want to do a smaller step by step approach to gain the needed
knowledge.
For that reason this PR does the most stupidest thing that can be done to
share memory between instances. Download the render result to CPU RAM share
the host pointer with the OpenXR instance which copies it to the swap chain.
Also the synchronization is done using wait idle commands.
<video src="attachments/32a0d69b-c3fa-4272-aea0-d207609afaaf" title="Screencast From 2025-03-18 11-16-17.webm" controls></video>
**Gaining knowledge**
- Experiment with `VK_KHR_external_memory_host` extension for uploading vertex buffers (not related to OpenXR).
- Import host pointer with `VK_KHR_external_memory_host`. This reduces the additional
memcpy on OpenXR side.
- Export host pointer from Blender side from a mappable buffer.
- Replace host pointers with fd/dmabuf/winhandle
- Remove mappable buffer.
Ref #133718
Pull Request: https://projects.blender.org/blender/blender/pulls/133824
3885a37541