Based on the AOMedia Film Grain Synthesis 1 (AFGS1) spec:
  https://aomediacodec.github.io/afgs1-spec/

The parsing has been changed substantially relative to the AV1 film
grain OBU. In particular:

1. There is the possibility of maintaining multiple independent film
   grain parameter sets, and decoders/players are recommended to pick
   the one most appropriate for the intended display resolution. This
   could also be used to e.g. switch between different grain profiles
   without having to re-signal the appropriate coefficients.

2. Supporting this, it's possible to *predict* the grain coefficients
   from previously signalled parameter sets, transmitting only the
   residual.

3. When not predicting, the parameter sets are now stored as a series of
   increments, rather than being directly transmitted.

4. There are several new AFGS1-exclusive fields.

I placed this parser in its own file, rather than h2645_sei.c, since
nothing in the generic AFGS1 film grain payload is specific to T.35, and
to compartmentalize the code base.