Add an API for a "keyframe has been requested" event

[alvestrand]

This is split from #207 - in the October interim discussion, the overall congestion control API was seen as requiring more discussion, but the keyframe proposal was thought reasonable to pursue on its own.

I preserved the split into "ScriptSource" and "ScriptSink" interfaces - I think that can be valuable for documenting which interfaces are facing in which direction on the transformer.

Comments welcome.

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Preview | Diff

[alvestrand]

Fixes #210

[alvestrand]

Should rewrite as "when the UA decides that a key frame has been requested, the UA will queue a task to fire an event...."

[jan-ivar]

LGTM if @youennf agrees.

[alvestrand]

Waiting for @youennf

[youennf]

Let's first introduce onkeyframerequest without introducing the mixins, we can do that as a follow-up.

[alvestrand]

Mixins removed as agreed during the editors meeting.
I also tightened up the definition of what to do when the keyframerequest event fires.
Should be ready to merge now - hope to merge it before the WG meeting Tuesday, so that we can say "it is done".

[alvestrand]

Despite bikeshed refs --for=Event --text='canceled flag' returning a value (spec=dom), I'm unable to make {{Event/canceled flag}} autolink. @dontcallmedom help?

[dontcallmedom]

the correct syntax would be [=Event/canceled flag=] - the {{ }} syntax is only for WebIDL terms, when "canceled flag" is a regular definition (see relevant Bikeshed doc)

[alvestrand]

the correct syntax would be [=Event/canceled flag=] - the {{ }} syntax is only for WebIDL terms, when "canceled flag" is a regular definition (see relevant Bikeshed doc)

Thanks - that worked. I could have sworn that I tried that before!

[dontcallmedom-bot]

This issue was discussed in WebRTC December 12 2023 meeting – 12 December 2023 (Keyframe API)