Fix regression for representationInfo's segmentDuration

Created by: bbert

This PR fixes a regression introduced in v3.0. Since v3.0, segments info lists are no more pre-computed, each segment info is now computed when requesting a segment. As a consequence, segment duration for any representation is not set until a media segment is requested. Since segmentDuration may be used prior to any media segment request (for example buffer target calculation for text tracks, or segment availability range computation), this leads to some dysfunction (for example text track switching for streams with SegmentTimeline).