public delegate FlowReturn CollectPadsBufferFunction (CollectPads pads, CollectData data, owned Buffer buffer)
A function that will be called when a (considered oldest) buffer can be muxed.
If all pads have reached EOS, this function is called with
the CollectPads that triggered the callback
the CollectData of pad that has received the buffer
user data passed to set_buffer_function
OK for success