Spec is unclear when and how WebCodecs VideoFrame objects are created

[youennf]

For MediaStreamTrackProcessor exposing camera content, implementations probably get information to create VideoFrame objects.
For MediaStreamTrackProcessor exposing VideoTrackGenerator content, the VideoFrame exposed by MediaStreamTrackProcessor is not the JS object written in VideoTrackGenerator but a clone.

We should describe this in the spec.
And we should make sure that video frames are correctly closed.