Create generic encoder/decoder infrastructure

[jackjansen]

It would be good if we could treat codecs the same we we do nowadays for capturers: allow multiple implementations, which each register themselves upon loading with the generic codec component.

User code would then normally use the generic encoder/decoder factories, and the parameter would include either the codec name or (maybe better) the 4CC (plus, maybe only for the encoder, the parameters).

The first two codecs we can implement are the current mpeg anchor codec and the serialised (which can be considered the "null codec" or "passthrough codec").

Next could be Draco #65

Next could be @zhouzhouha 's vpcc work.

This would also make life simpler for #130.