enable pass-through on visionOS

This is technically possible already by setting the background to transparent and setting the initial immersion mode to "UIImmersionStyleMixed".

<dict>
    <key>UISceneConfigurationName</key>
    <string>ImmersiveApp</string>
    <key>UISceneInitialImmersionStyle</key>
    <string>UIImmersionStyleMixed</string>
</dict> 

What's missing is to have some sensible way to detect, report and/or change the mode at runtime, as is the case with OpenXR.