Add experimental fallback codec support (#334)
* add experimental polycast encodings
* remove unneeded null check
* rename to multiCodecSimulcast
* capitalize
* wip
* wip
* use backupCodec as indicator for multi codec simulcast
* use backup codec options
* add default backup encoding
* make sure same codec as backup has no effect
* use same bitrate as vp8 for h264
* remove vp9 reference and account for #397
* remove comments
* remove comments
* changeset
