perf: Remove promises from getAllThumbnails loop (#8118)

`getAllThumbnails` creates an array of promises when grabbing each
thumbnail. This is because `getThumbnails` might have to create a
segment index, but `getAllThumbnails` already does this. This separates
`getThumbnails` into two functions so `getAllThumbnails` can get the
thumbnails synchronously.

Author: dpfister2

lib/player.js

@@ -4394,7 +4394,7 @@ shaka.Player = class extends shaka.util.FakeEventTarget {
     if (!imageStream.segmentIndex) {
       await imageStream.createSegmentIndex();
     }
-    const promises = [];
+    const thumbnails = [];
     imageStream.segmentIndex.forEachTopLevelReference((reference) => {
       const dimensions = this.parseTilesLayout_(
           reference.getTilesLayout() || imageStream.tilesLayout);
@@ -4403,12 +4403,16 @@ shaka.Player = class extends shaka.util.FakeEventTarget {
         const duration = reference.trueEndTime - reference.startTime;
         for (let i = 0; i < numThumbnails; i++) {
           const sampleTime = reference.startTime + duration * i / numThumbnails;
-          promises.push(this.getThumbnails(trackId, sampleTime));
+          const thumbnail =
+              this.getThumbnailsByStream_(
+              /** @type {shaka.extern.Stream} */ (imageStream), sampleTime);
+          if (thumbnail) {
+            thumbnails.push(thumbnail);
+          }
         }
       }
     });
-    const thumbnails = await Promise.all(promises);
-    return thumbnails.filter((t) => t);
+    return thumbnails;
   }
 
   /**
@@ -4465,6 +4469,19 @@ shaka.Player = class extends shaka.util.FakeEventTarget {
     if (!imageStream.segmentIndex) {
       await imageStream.createSegmentIndex();
     }
+
+    return this.getThumbnailsByStream_(imageStream, time);
+  }
+
+  /**
+   * Return a Thumbnail object from a image stream and time.
+   *
+   * @param {shaka.extern.Stream} imageStream
+   * @param {number} time
+   * @return {?shaka.extern.Thumbnail}
+   * @private
+   */
+  getThumbnailsByStream_(imageStream, time) {
     const referencePosition = imageStream.segmentIndex.find(time);
     if (referencePosition == null) {
       return null;