- Add [CAMetalLayer screenMVK] extension method to retrieve screen.
- If the layer has a delegate view, use it to locate the screen, otherwise
revert to iterating across windows, looking for the CAMetalLayer, to identify
the window the layer is in, from which to retrieve the screen.
- Fix the iteration of the layer hierarchy, to accommodate that Apple may
add superlayers to the CAMetalLayer under the covers.
- Update MoltenVK_Runtime_UserGuide.md to encourage the app to ensure the view
is the delegate of the CAMetalLayer, to more efficiently access the screen.