Handling of depth buffers for stereoscopic systems

Splitting @cabanier's question into a new issue:

How is a single depth buffer going to work with stereo? Will each eye have its own depth buffer?

The way I thought about it is that XRDepthInformation that we return must be relevant to the XRView that was used to retrieve it. In case of a stereo system w/ only one depth buffer, there would be 2 options: either reprojecting the buffer so that each of XRViews gets the appropriate XRDepthInformation, or exposing an additional XRView that would be used only to obtain the single depth buffer (but then it'd be up to the app to reproject, there are some XRViews for which XRDepthInformation would be null, and we are creating a synthetic XRView so maybe not ideal). If we were to require the implementation to reproject the depth buffer, how big of a burden would that be?

Start with explainer.md and the linked question in depth-sensing issue #2, then review the specification's wording for XRDepthInformation and XRView. Done requires a resolved design for stereo systems with a single depth buffer and corresponding specification changes, but the issue does not identify a concrete implementation or test entry point.