Support video transparency

[tidoust]

In WebRTC and WebCodecs. Only an implementation issue? Are there additional hooks that need to be exposed at the API level?

Raised in:

• It's time to WHIP WebRTC into shape

[chcunningham]

For WebCodecs, we have EncodedVideoChunkMetadata.alphaSideData, which will be populated with alpha info on a per-chunk basis when VideoEncoderConfig.alpha is set to keep.

https://w3c.github.io/webcodecs/#ref-for-dom-encodedvideochunkmetadata-alphasidedata

Buuuut, for Chrome, we don't actually have any encoders currently that support VideoEncoderConfig.alpha = "keep" (default is "discard").

[chcunningham]

Chrome bug here
https://bugs.chromium.org/p/chromium/issues/detail?id=1195433