Creates a stream-id for the source gstreamer.Pad pad by combining the upstream information with the optional stream_id of the stream of pad. pad must have a parent gstreamer.Element and which must have zero or one sinkpad. stream_id can only be NULL if the parent element of pad has only a single source pad.
This function generates an unique stream-id by getting the upstream stream-start event stream ID and appending stream_id to it. If the element has no sinkpad it will generate an upstream stream-id by doing an URI query on the element and in the worst case just uses a random number. Source elements that don't implement the URI handler interface should ideally generate a unique, deterministic stream-id manually instead.