Support Center Stage on Apple devices

For pure Web we’re screwed until WebRTC constraints support center stage - but when we ship a native version it would be fun to wedge in a center state button (at the expense of having to ship our own webrtc and web view…).

Thisnis P5, blue sky, needless to say

https://developer.apple.com/documentation/avfoundation/avcapturedevice/3738418-centerstagecontrolmode