Add face detection mode constraint

[eehakkin]

This spec update allows face detection as descibed in #289.

The changes include a new face detection mode constrainable property and a new detectedFaces sequence which is accessible via a new asynchronous getAttributes() accessor.

This allows following kind of code to be used for face detection:

<script>
// Check if face detection is supported by the browser.
const supports = navigator.mediaDevices.getSupportedConstraints();
if (supports.faceDetectionMode) {
    // Browser supports camera face detection.
} else {
    throw('Face detection is not supported');
}

// Open camera with face detection enabled and show to user.
const stream = await navigator.mediaDevices.getUserMedia({
    video: {faceDetectionMode: "simple"}
});
const video = document.querySelector("video");
video.srcObject = stream;

// Get face detection results for the latest frame
const videoTracks = stream.getVideoTracks();
const videoTrack = videoTracks[0];
const settings = videoTrack.getSettings();
if (settings.faceDetectionMode && settings.faceDetectionMode != "off") {
    const attributes = await videoTrack.getAttributes();
    for (const face of attributes.detectedFaces) {
        console.log(
         ` Face @ (${face.boundingBox.x}, ${face.boundingBox.y}),` +
         ` size ${face.boundingBox.width}x${face.boundingBox.height}`);
    }
}
</script>

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Preview | Diff

[eehakkin]

Closing in favor of w3c/mediacapture-extensions#48.