Make use of RTCRtpScriptTransform if available.

[youennf]

Use VP8 by default as it works best with encryption transforms.

Fixes #1467

[youennf]

@fippo,PTAL.

[youennf]

Interestingly, the JS encryption code seems to run much faster on Chrome than STP. This might require some investigations on Safari side. I filed https://bugs.webkit.org/show_bug.cgi?id=229190 to keep track of it.

[youennf]

linter does not like 'new RTCRtpScriptTransform', not sure what is the best way to address this. Any idea?

[fippo]

/* global RTCRtpScriptTransform */ at the top of the file should help. There is a followup to teach eslint about this but I never figured out who adds new globals to it.

I'll defer to @alvestrand for review

[youennf]

Thanks @fippo, linter is good now.
@alvestrand PTAL.

[alvestrand]

Nice illustration of the relatively small differences between the approaches when you have decided to use a worker.

No big comments.