Support more obscure channel layouts by adding a new mapping family

As per https://trac.ffmpeg.org/ticket/5718#comment:29 , there are a lot of other surround layouts that are not directly representable by libopus. A common one is 5.1(side), but I ran into a soundtrack encoded in 4.0 surround today, and there are lots of other more obscure layouts out there that are still common enough to be a headache.

My strawman proposal is to add a mapping family #4 that lists most all the other channel layouts that are out there, and some type of list in libopus to define the attributes of these channel layouts.