Buffers would always start with timestamp 0 and we'd start streaming
from the first buffer, but live streams always start streaming from
the last fragment - 3 fragments in the playlist, which makes its
timestamp, as returned by get_next_fragment, be whatever position
they had in the playlist. This makes sure the position correctly
reports the position of the buffer in the playlist, and added a shifting
variable to allow seeking in the middle of fragments.
By using a separate variable, first it allows us to sort the lists
of alternates but keep the pointer on the first occurence in the main
playlist (to respect the spec of starting with the bitrate specified
first in the main playlist). It also avoid playing with the lists variable
which should be used to store the list of playlists and not as a pointer
to the current one.
Also fixes a memleak with the g_list_foreach freeing the lists, if it wasn't
pointing to the first element of the list.
