Face Detection: How metadata should be tied to requestVideoFrameCallback

[youennf]

rvfc can be used to grab information of video frames and do canvas painting. FaceDetection metadata can be useful in that context. There can be different approaches to how we could expose FaceDetection metadata in that context:

1. Extend https://wicg.github.io/video-rvfc/#dictdef-videoframemetadata with an optional faceDetection object.
2. Have the callback take a third additional parameter that would contain FaceDetection metadata (and potentially more).
3. Create a new rvfc dedicated to FaceDetection