Support for Screen Content Coding

[aboba]

Currently there is no parameter defined in the AV1 registry to enable or force screen content coding (e.g. set seq_force_screen_content_tools to SELECT_SCREEN_CONTENT_TOOLS).

Reference: AV1 Bitstream Spec page 31

[Djuffin]

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

[chrisn]

Minutes from 11 Apr 2023 Media WG meeting: https://www.w3.org/2023/04/11-mediawg-minutes.html#t02