Clarify that all VideoFrameMetadata members are optional

[jan-ivar]

The spec says:

dictionary VideoFrameMetadata {
  // Possible members are recorded in the VideoFrame Metadata Registry.
};

But the [webcodecs-video-frame-metadata-registry] does not say whether members are optional or required.

Since dictionary members are optional by default, readers will hopefully infer (correctly) that they're optional, since neither spec mentions required.

Readers might also infer this from VideoFrameInit where metadata is not required, if I'm not mistaken.

Still, it might be good to call this out, since a single required member would mean the VideoFrame constructor would suddenly require this input and no longer be backwards compatible.

[jan-ivar]

This also becomes important as we specify members and their rules for when they should be present or not. VideoFrames can come from different sources, and it currently seems unclear whether all VideoFrames regardless of source are to present with the same metadata members or not.

Since the VideoFrame constructor already lets JS create frames with a subset of metadata, should we suggest a model where each source only provides the metadata relevant to it?

To me, this seems likely to happen anyways. E.g. VideoFrames from cameras might have {backgroundBlur: true|false}, but VideoFrames created from canvas might not.