fix(DASH): Fix bad segment URLs in SegmentTimeline

When Representations have different BaseURLs, segment URLs generated
for SegmentTimeline were always using the BaseURL of the last
Representation because the wrong parsing context was being used.  This
fixes the issue and adds a regression test.

This only affected v3.0.0.

Closes shaka-project#2650

Change-Id: I04df950b5d3205e102f9c74f52ece9773ce92282

lib/dash/segment_template.js

@@ -90,7 +90,7 @@ shaka.dash.SegmentTemplate = class {
       }
 
       const references = SegmentTemplate.createFromTimeline_(
-          context, info, initSegmentReference);
+          shallowCopyOfContext, info, initSegmentReference);
 
       const periodStart = context.periodInfo.start;
       const periodEnd = context.periodInfo.duration ?

test/dash/dash_parser_manifest_unit.js

@@ -1583,4 +1583,49 @@ describe('DashParser Manifest', () => {
     expect(variant.audio).toBeTruthy();
     expect(variant.video).toBeTruthy();
   });
+
+  // Regression #2650 in v3.0.0
+  // A later BaseURL was being applied to earlier Representations, specifically
+  // in the context of SegmentTimeline.
+  it('uses the correct BaseURL for SegmentTimeline', async () => {
+    const manifestText = [
+      '<MPD type="static">',
+      '  <Period id="1" duration="PT30S">',
+      '    <AdaptationSet id="2" mimeType="video/mp4">',
+      '      <SegmentTemplate media="$Number$.mp4" startNumber="1">',
+      '        <SegmentTimeline>',
+      '          <S t="0" d="30" />',
+      '        </SegmentTimeline>',
+      '      </SegmentTemplate>',
+      '      <Representation id="video-sd" width="640" height="480">',
+      '        <BaseURL>http://example.com/r0/</BaseURL>',
+      '      </Representation>',
+      '      <Representation id="video-hd" width="1920" height="1080">',
+      '        <BaseURL>http://example.com/r1/</BaseURL>',
+      '      </Representation>',
+      '    </AdaptationSet>',
+      '  </Period>',
+      '</MPD>',
+    ].join('\n');
+
+    fakeNetEngine.setResponseText('dummy://foo', manifestText);
+
+    /** @type {shaka.extern.Manifest} */
+    const manifest = await parser.start('dummy://foo', playerInterface);
+
+    const video0 = manifest.variants[0].video;
+    await video0.createSegmentIndex();
+    goog.asserts.assert(video0.segmentIndex, 'Null segmentIndex!');
+    const segment0 = Array.from(video0.segmentIndex)[0];
+    const uri0 = segment0.getUris()[0];
+
+    const video1 = manifest.variants[1].video;
+    await video1.createSegmentIndex();
+    goog.asserts.assert(video1.segmentIndex, 'Null segmentIndex!');
+    const segment1 = Array.from(video1.segmentIndex)[0];
+    const uri1 = segment1.getUris()[0];
+
+    expect(uri0).toBe('http://example.com/r0/1.mp4');
+    expect(uri1).toBe('http://example.com/r1/1.mp4');
+  });
 });