Define order of AdaptationSet preference

[joeyparrish]

When a piece of content has multiple viable AdaptationSets with the same tuple of {contentType, language, role}, the first one is chosen arbitrarily. For example, there may be two AdaptationSets of {'video', null, null} (one H.264, one VP9) or two of {'audio', 'en-US', null} (one Vorbis, one AAC) or (conceivably, but not seen in the wild) two of {'text', 'fr', 'caption'} (one TTML, one WebVTT).

We should define some ordering of equivalent AdaptationSets for browsers that support multiple codecs and containers, such as Firefox and Chrome, both of whom support MP4 and WebM.

In Shaka Player's sample media, many assets contain both VP9 and H.264, with significant bandwidth savings in VP9. We should define a way to order AdaptationSets so that we prefer those which save bandwidth for equivalent resolutions.

Although we are discussing AdaptationSets for DASH at the moment, the problem is more general, and the solution should be applied outside any particular manifest parser after conversion to StreamSets. This ensures that the fix applies to future manifest parsers and does not interfere with any grouping caused by DASH's AdaptationSetSwitching properties.

Proposed order of preference when language and role are the same:

1. StreamSet with highest top resolution (video only)
   • example: A at 480p, 720p, 1080p vs B at 480p, 720p, 1080p, 2160p (business logic may require the use of certain codecs for certain resolutions), choose B
   • example: A at 240p (old codec, very low bitrate, for certain devices with no other codecs) vs B at 480p (modern codec, higher bitrate and much higher quality, for all other devices), choose B
2. StreamSet with lowest average bandwidth across all Streams (to prefer VP9 or H.265 over VP8 or H.264 without hard-coding preferences for any of them)
3. StreamSet 0 (arbitrary choice)

This change would take place in shaka.util.StreamUtils.chooseStreamSets.