OpenXR: Add support for visibility mask

[BastiaanOlij]

This PR adds a new OpenXR node that, if supported by the XR runtime, renders a mask that covers the parts of the screen that are never visible to the user due to lens distortion. This prevents costly fragments being rendered in those areas and should lead to a small performance boost.

It's simply a matter of adding the node as a child node of the camera:

The end result is visible in the preview:

Todos:

• Need to fix thread check when we're not using multi threading
• Need to ensure this geometry is rendered first, esp on Mobile where we don't use a depth prepass. TBDR helps here but can't be relied upon on desktop.
• Need an icon (help please!)
• Maybe add a switch in OpenXR project settings?