FragmentLoader : ensure we end up loading the right fragment after PTS analysis in case of different fragment duration amongst quality level
FragmentLoader : ensure we end up loading the right fragment after PTS analysis in case of different fragment duration amongst quality level

use SN(last PTS) to compute next SN, instead of SN(first PTS)+1
previously, in case we had two levels, with frag duration as follow:
level 0 = 10s, 10s, 10s
level 1 =  5s,  5s,  5s, 5s, 5s, 5s

and flashls was switching from level 0 to level 1, with first loaded fragment being SN 0, level 0,
then we were picking up SN 1 level 1 instead of SN 2 level 1.

hola/flashls,vidible/vdb-flashls,fixedmachine/flashls,tedconf/flashls,clappr/flashls,hola/flashls,jlacivita/flashls,tedconf/flashls,neilrackett/flashls,codex-corp/flashls,vidible/vdb-flashls,neilrackett/flashls,clappr/flashls,codex-corp/flashls,mangui/flashls,NicolasSiver/flashls,NicolasSiver/flashls,fixedmachine/flashls,jlacivita/flashls,mangui/flashls