Enable configuration of AV1 screen content coding tools

[aboba]

Fix for #646
Fix for #478

Rebase of #652

[Djuffin]

Is it really a per frame setting?
It seems that #652 does the same but on the VideoEncoderConfig level

[aboba]

During a screen sharing session, the content can change so that screen content coding might no longer be advisable. For example, a slide presentation could include an embedded video, or a user could shift to sharing another tab. libaom supports applying screen content coding on a per-frame basis, so having it applied per-frame in WebCodecs may make sense.

[Djuffin]

WebCodecs Editors decision: let's put it into VideoEncoderConfig so VideoEncoder can explicitly refuse it if it's not supported.

[aboba]

PR redone to make it an encoder configuration option, as discussed in the Editor's meeting. This still allows screen content coding to be turned on/off in mid-stream by calling encoder.configure().

[aboba]

Discussed at 14 April 2023 MEDIA WG meeting: https://lists.w3.org/Archives/Public/public-media-wg/2023Apr/0007.html

[aboba]

Editor's meeting: merge it!

[fippo]

Late comment but since this hasn't shipped yet... can we stop saying this is "forced" and instead say it gets enabled (i.e. enableScreenContentTools)?
Forcing may have implications e.g. when you try to do "force" it on a hardware encoder where it is not supported. Does that imply it should fail or should this simply not get enabled.

Wordsmithing ;-)

[fippo]

(force... is used by the AV1 spec so keeping this consistent is good)