fix(DASH): Fix bufferBehind with image tracks regression (shaka-proje…

…ct#5210)

A recent change to allow for on-demand segment references (shaka-project#5061) ended up causing a
regression with an older issue (shaka-project#4717), where the presence of an image track overrides
`bufferBehind` behavior.
This fixes the segment template handler to properly account for image tracks, once again.

lib/dash/segment_template.js

@@ -143,7 +143,7 @@ shaka.dash.SegmentTemplate = class {
         tsi.evict(availabilityStart);
       }
 
-      if (info.timeline) {
+      if (info.timeline && context.adaptationSet.contentType !== 'image') {
         const timeline = info.timeline;
         context.presentationTimeline.notifyTimeRange(
             timeline,