Set a priori codec preferences

[martinthomson]

Agreed at TPAC that an ordered list of codecs can be set on RTCRtpTransceiver. This only determines the ordering of the codecs on the corresponding media section generated by createOffer or createAnswer. Respecting these is down to the normal SDP rules for codec preferences.

Note that the values that are set should persist. However, we won't provide an accessor for this.

@juberti notes that we might also want to push certain codecs to the bottom: CN, telephone-event, and all FEC payload types. That might be best to avoid accidental interoperability issues.

[aboba]

One thing that needs to be nailed down is whether only one codec in the codec list can be sending at a time (e.g. whether encodings[i].active can be set to true for only one value of i).

We've already seen differing interpretations of this within the ORTC API.

[martinthomson]

@aboba, I think that's a separable issue.

[elagerway]

Closing this as PR #361 was merged.