visionOS performance parity with OpenXR

To ensure we get good and similar performance across the platforms we support we need to implement the same techniques on all of them. Currently OpenXR has some extra features enabled (assuming the device supports it) that is not implemented for visionOS, even though a lot of the ground work has already been done to ensure it's possible; for example multiview support in QRhi.

• Enable multiview
• foveated rendering
• Threaded rendering
• Pass-through