RE: [webrtc-pc] Plan X: Add an API for using RID to do simulcast (#353)

Mathieu said:

The reasoning for having a maxFramerate (possibly in addition to scaleBy, which is what I remember we actually converged to), is that for screen sharing, you wouldn't want to skip every other slide if you have a framerateScale of 2.


[BA] In SDP O/A it is possible to specify max-fr and max-fs for each simulcast stream.  While these represent capabilities of the decoder, presumably a recipient of these values will take them into account when sending.  So the question is whether a maxFramerate parameter is used just to influence the max-fr value that issues in createOffer(), or whether it interacts with the SDP O/A max-fr parameter in some way.