Previously hlsdemux wasn't sending out any newsegment.
Here we push a GST_FORMAT_TIME newsegment, and whenever possible we
try to indicate the proper start time.
This allows downstream elements to relay the start/time values properly
to the sinks, allowing better stream switching.
When switching bitrates, we might end up switching to a different
media-type (like from aac to/from mpeg-ts).
For this switch to behave properly in decodebin2, this patch adds:
* dynamic source pads (which will be added/removed whenever a stream
media type changes
* re-checking the fragment media type whenever we switch to a different
playlist
