Fix multiperiod playback

Lookup times when transitioning from one Period to another, need to be
relative to the presentation, not the next period.

I'm not sure when this broke.

Relates to #310

Change-Id: I6e3f94bb9c6aad8c8f3ef73e73b0073e015f796e

lib/media/streaming_engine.js

@@ -977,9 +977,11 @@ shaka.media.StreamingEngine.prototype.getSegmentReference_ = function(
     } else {
       // Something is buffered from another Period.
       shaka.log.v1(logPrefix, 'next position unknown: another Period buffered');
+      var lastPeriodStart =
+           this.manifest_.periods[mediaState.lastSegmentPeriodIndex].startTime;
       position = this.lookupSegmentPosition_(
           mediaState,
-          mediaState.lastSegmentReference.endTime,
+          lastPeriodStart + mediaState.lastSegmentReference.endTime,
           currentPeriodIndex);
     }
   }
@@ -1006,7 +1008,7 @@ shaka.media.StreamingEngine.prototype.getSegmentReference_ = function(
  * Looks up the position of the next segment needed.
  *
  * @param {shaka.media.StreamingEngine.MediaState_} mediaState
- * @param {number} time
+ * @param {number} time relative to presentation timeline
  * @param {number} currentPeriodIndex
  * @return {number}
  * @throws {!shaka.util.Error} If the next segment does not exist.
@@ -1029,6 +1031,7 @@ shaka.media.StreamingEngine.prototype.lookupSegmentPosition_ = function(
   if (position == null) {
     shaka.log.warning(logPrefix,
                       'next segment does not exist:',
+                      'lookupTime=' + lookupTime,
                       'time=' + time,
                       'currentPeriod.startTime=' + currentPeriod.startTime,
                       'mediaState.drift=' + mediaState.drift);