Return a constantly incrementing sequence number.
This function is used internally to GStreamer to be able to determine which events and messages are "the same". For example, elements may set the seqnum on a segment-done message to be the same as that of the last seek event, to indicate that event and the message correspond to the same segment.
This function never returns SEQNUM_INVALID (which is 0).
A constantly incrementing 32-bit unsigned integer, which might overflow at some point. Use seqnum_compare to make sure you handle wraparound correctly.