Fix gaps when switching Streams.
Fix gaps when switching Streams.

When a Stream changes a new segment position must be looked up,
as segments between Streams may not be aligned.

Change-Id: I449e36e21c90e066713a2b253ab8c2f76f600769

TheModMaker/shaka-player,TheModMaker/shaka-player,tvoli/shaka-player,TheModMaker/shaka-player,brightcove/shaka-player,baconz/shaka-player,indiereign/shaka-player,baconz/shaka-player,vimond/shaka-player,ustudio/shaka-player,tvoli/shaka-player,shaka-project/shaka-player,russitto/shaka-player,ustudio/shaka-player,russitto/shaka-player,TheModMaker/shaka-player,TobbeEdgeware/shaka-player,indiereign/shaka-player,brightcove/shaka-player,Ross-cz/shaka-player,TobbeEdgeware/shaka-player,TobbeEdgeware/shaka-player,ustudio/shaka-player,baconz/shaka-player,vimond/shaka-player,indiereign/shaka-player,Ross-cz/shaka-player,vimond/shaka-player,shaka-project/shaka-player,shaka-project/shaka-player,brightcove/shaka-player,tvoli/shaka-player,Ross-cz/shaka-player,ustudio/shaka-player,tvoli/shaka-player,shaka-project/shaka-player,russitto/shaka-player