dash: in segment timeline parsing, compute start time in case we need it afterward (manifest forwarding)
dash: in segment timeline parsing, compute start time in case we need it afterward (manifest forwarding)

rbouqueau/gpac,rbouqueau/gpac,rbouqueau/gpac,rbouqueau/gpac,gpac/gpac,rbouqueau/gpac,gpac/gpac,rbouqueau/gpac,gpac/gpac,gpac/gpac,rbouqueau/gpac,rbouqueau/gpac,gpac/gpac,gpac/gpac,gpac/gpac,gpac/gpac