video-audio-caption download correlation

Have you read the FAQ and checked for duplicate issues:yes

What version of Shaka Player are you using:v2.1.6

Can you reproduce the issue with our latest release version:yes

Can you reproduce the issue with the latest code from master:

Are you using the demo app or your own custom app:demo

If custom app, can you reproduce the issue using our demo app:

What browser and OS are you using:chrome/ff

What are the manifest and license server URIs: (you can send the URIs to [e-mail] instead, but please use GitHub and the template for the rest)

**What did you do?**opened stream and waited

What did you expect to happen? stream must play as fast as possible

What actually happened? several seconds to wait additionally

I throttled chrome to low network speed and saw that palyer downloaded segments in this way: video-fragment-1(downloaded) audio-fragment-1(downloaded) caption-fragment-1(downloaded) audio-fragment-2(downloaded) caption-fragment-2(downloaded) audio-fragment-3(downloaded) caption-fragment-3(downloaded) audio-fragment-4(downloaded) video-fragment-2(still downloading) caption-fragment-4(downloaded) audio-fragment-5(downloaded) caption-fragment-5(downloaded)

So video/audio/caption downloading is not correllated. If player have no video, why downlaod audio and caption? The same problem i described at https://github.com/Dash-Industry-Forum/dash.js/issues/1319 Can you make same fix here? It will be very helpful for low speed users.

[joeyparrish]

Fix cherry-picked to v2.3.5.

[joeyparrish]

I think this is feasible. Tentatively scheduled for v2.3.