Fix for #1150 and improvements for #1143

[ashnfb]

@cobarx @ahmetabdi
Fixes #1150
Improves #1143 (onLoad -> textTracks returns only valid sideloaded vtt)

[cobarx]

@ashnfb Thanks for all your contributions!

Did you by any chance figure out how to disable sideloaded tracks? I'm still waiting to hear back from my contact.

[ashnfb]

@cobarx I gave disabling tracks a try. I'm a little frustrated with the lack of AVPlayer docs. I'll try and spend a bit more time on it.

[cobarx]

@ashnfb Yup, AVPlayer is great when it works, when it doesn't you're just out of luck.

I was thinking about this last night and I don't think we want the fallback code if it can't find the requested track. The way that code works right now is it will only use the fallback if the system Captions preference is enabled, so the behavior will be inconsistent. I think it's totally reasonable to say that if you request an invalid track nothing happens. Really it should throw an error, but there's no framework for it (onError has already been documented as only being for load errors). Also, we don't use the fallback code on Android, so the behaviors are different. Do you have an objection to me reverting that part of the code?

[ashnfb]

@cobarx no objection to reverting that code!