Allow not specifying scaleResolutionDownTo if encoding is inactive

[henbos]

The text currently says to throw if "scaleResolutionDownTo is not specified on all encodings", but this is overly restricting, it should instead say "all active encodings".

It is common for an app to use the sender for both singlecast and simulcast so {720p,inactive,inactive} is a common use case and it doesn't really make sense to force the app to put in dummy scaleResolutionDownTo values on the inactive layers in order to avoid an exception. Restricting the resolution of an inactive encoding is a NO-OP so it couldn't possibly matter which value you plug in there, therefore it should be allowed to be missing.