webrtc-pc

9cadefe1 - Specify which encoding parameters can be set through sendEncodings.

Specify which encoding parameters can be set through sendEncodings. Fixes #1073. All read-write parameters and `rid` can be set, but the rest are ignored, as decided in the April virtual interim. Also added some detail to `getParameters` description, explaining that it should return the previously provided `sendEncodings` even if `setRemoteDescription` hasn't been called yet. I'm not yet addressing the simulcast error point that arose in the discussion of the issue. We can address that after we finish fleshing out how simulcast errors will be handled in the first place.