tsdemux: Expose new pads if got new stream-start
Submitted by Seungha Yang
Link to original bug (#777088)
Description
If upstream pushes new stream-start event with updated stream-id,
nobody can guarantee that newly incoming stream is identical to the previous one.
So, demux should expose new pads whatever previously demux has.
This is to fix the case that codec/stream configuration change
with hls bitrate switching.