fix: Load image streams for offline multiperiod

When loading offline content, we should load the image streams as
well.

Issue shaka-project#3383

Change-Id: I8cdd3d1ade188b7b2278a0eac5510a968229c6fc

Author: joeyparrish

lib/util/periods.js

@@ -399,6 +399,8 @@ shaka.util.PeriodCombiner = class {
         (streams) => streams.filter((s) => s.type == ContentType.VIDEO));
     const textStreamDbsPerPeriod = streamDbsPerPeriod.map(
         (streams) => streams.filter((s) => s.type == ContentType.TEXT));
+    const imageStreamDbsPerPeriod = streamDbsPerPeriod.map(
+        (streams) => streams.filter((s) => s.type == ContentType.IMAGE));
 
     // It's okay to have a period with no text, but our algorithm fails on any
     // period without matching streams.  So we add dummy text streams to each
@@ -429,6 +431,13 @@ shaka.util.PeriodCombiner = class {
         shaka.util.PeriodCombiner.cloneStreamDB_,
         shaka.util.PeriodCombiner.concatenateStreamDBs_);
 
+    const combinedImageStreamDbs = await shaka.util.PeriodCombiner.combine_(
+        /* outputStreams= */ [],
+        imageStreamDbsPerPeriod,
+        /* firstNewPeriodIndex= */ 0,
+        shaka.util.PeriodCombiner.cloneStreamDB_,
+        shaka.util.PeriodCombiner.concatenateStreamDBs_);
+
     // Recreate variantIds from scratch in the output.
     // HLS content is always single-period, so the early return at the top of
     // this method would catch all HLS content.  DASH content stored with v3.0
@@ -457,7 +466,8 @@ shaka.util.PeriodCombiner = class {
 
     return combinedVideoStreamDbs
         .concat(combinedAudioStreamDbs)
-        .concat(combinedTextStreamDbs);
+        .concat(combinedTextStreamDbs)
+        .concat(combinedImageStreamDbs);
   }
 
   /**