mpegtsmux: set group-id in stream-start event

Submitted by Aurélien Zanelli

Link to original bug (#740124)

Description

Currently, mpegts muxer create a new STREAM_START event and push it downstream without setting group_id causing a basesink derived element to output following FIXME message:

0:00:00.044971265 7168 0x92d8a60 FIXME basesink gstbasesink.c:3064:gst_base_sink_default_event:<fakesink0> stream-start event without group-id. Consider implementing group-id handling in the upstream elements

I propose to set a new group_id using helper before sending stream-start event.

This FIXME will also happen with others muxer, I can fix them if the solution is correct.