Draft: livesync: add support for variable framerate
A variable framerate is signaled by a framerate of 0/1 and the max-framerate field. This may be the result of source that can produce up to max-framerate frames, but may produce less. A live stream may expect frames with the expected max-framerate and updated timestamps even if the frame contents didn't change.
Support streams with variable framerate in livesync and send frames with max-framerate to downstream.