Intent to implement: HTMLVideoElement's requestVideoFrameCallback

[Andrew Osmond]

Summary: Allows web authors to be notified when a video frame has been presented for composition.

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1800882

Specification: https://wicg.github.io/video-rvfc

Platform coverage: All

Preference: media.rvfc.enabled

DevTools bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1908398

Other browsers: Shipped in Safari 15.4 (2022), Chrome 83 (2020)

Standards position discussion: https://github.com/mozilla/standards-positions/issues/250

web-platform-tests: https://wpt.fyi/results/?label=master&label=experimental&aligned&q=video-rvfc