OTSubscriberDelegate extends OTSubscriberKitDelegate with additional
optional callbacks specific to the OTSubscriber implementation.
Currently, there is only one additional callback, but others may be
added in a future release.
Sent when a frame of video has been decoded. Although the
subscriber will connect in a relatively short time, video can take
more time to synchronize. This message is sent after the
[OTSubscriberKitDelegate subscriberDidConnectToStream:] message is sent.