[mediacapture-image] Extend `PhotoSettings` to support points/regions of interest for e.g. Auto Focus/Exposure

miguelao has just created a new issue for 
https://github.com/w3c/mediacapture-image:

== Extend `PhotoSettings` to support points/regions of interest for 
e.g. Auto Focus/Exposure ==

[`PhotoSettings`](https://w3c.github.io/mediacapture-image/#PhotoSettings)
  can specify e.g. `FocusMode` and `ExposureMode` but not where in the
 current scene to use as seeds for either of these.  For example 
Android provides [regions for 
autofocus](https://developer.android.com/reference/android/hardware/camera2/CaptureRequest.html#CONTROL_AF_REGIONS)
 and for 
[autoexposure](https://developer.android.com/reference/android/hardware/camera2/CaptureRequest.html#CONTROL_AE_REGIONS)),
 and so does Mac AVFoundation ([autofocus point of 
interest](https://developer.apple.com/library/ios/documentation/AVFoundation/Reference/AVCaptureDevice_Class/#//apple_ref/occ/instp/AVCaptureDevice/focusPointOfInterest)
 and [autoexposure point of 
interest](https://developer.apple.com/library/ios/documentation/AVFoundation/Reference/AVCaptureDevice_Class/#//apple_ref/occ/instp/AVCaptureDevice/exposurePointOfInterest)).
 Consider adding provisions in `PhotoSettings` to indicate which 
point(s) or region(s) to use for these operations (probably in a 
normalized framework, i.e. coordinates ranging in `[0,1)` )

Please view or discuss this issue at 
https://github.com/w3c/mediacapture-image/issues/44 using your GitHub 
account