Move rendering to a dedicated thread on visionOS

This is a strong recommendation (requirement) on visionOS. We've been able to get away with it so far, but this needs to be done. There's some roadblocks that needs to be cleared for this to work though, like:

• Multiview support on visionOS is required, as we for example cannot really on syncing on the render thread...
• Right now some communication between backend and front-end happens during rendering in ways that won't be safe or even work in a threaded environment.

Once this is done we should evaluate if the same should be done for OpenXR.