public void set_group_id (uint group_id)
All streams that have the same group id are supposed to be played together, i.e. all streams inside a container file should have the same group id but different stream ids.
The group id should change each time the stream is started, resulting in different group ids each time a file is played for example.
Use group_id_next to get a new group id.
a stream-start event
the group id to set