Handle SVC compatibility with Safari and Chrome prior to M113 (#1079)
* Handle SVC compatibility with Safari and Chrome prior to M113
Before M113 in Chrome, defining multiple encodings with an SVC codec indicated
that SVC mode should be used. Safari still works this way.
This is a bit confusing but is due to how libwebrtc interpreted the encodings field
before M113.
Announced here: https://groups.google.com/g/discuss-webrtc/c/-QQ3pxrl-fw?pli=1