Adaptive demuxers need to start downloading from specific positions
(fragments) for every stream, this means that all streams can snap-seek
to a different position when requested. Snap seeking in this case will
be done in 2 steps:

1) do the snap seeking on the pad that received the seek event and
   get the final position

2) use this position to do a regular seek on the other streams to
   make sure they all start from the same position

More arguments were added to the stream_seek function, allowing better control
of how seeking is done. Knowing the flags and the playback direction allows
subclasses to handle snap-seeking.
And also adds a new return parameter to inform of the final
selected seeking position that is used to align the other streams.

https://bugzilla.gnome.org/show_bug.cgi?id=759158