hlsdemux: Don't use approximate duration for fragment buffer pts

hlsdemux: Don't use approximate duration for fragment buffer pts

The duration values in playlists are approximate only, and for
playlist versions 2 and older they are only rounded integer values.
They cannot be used to timestamp buffers.  This resulted in playback
gaps and skips because the actual duration of fragments is slightly
different.  The solution is to only set the pts of the very first
buffer processed, not for each fragment.