avcodec/aom_film_grain: implement AFGS1 parsing

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.

libavcodec/aom_film_grain.h

@@ -30,9 +30,22 @@
 
 #include "libavutil/film_grain_params.h"
 
+typedef struct AVFilmGrainAFGS1Params {
+    int enable;
+    AVFilmGrainParams sets[8];
+} AVFilmGrainAFGS1Params;
+
 // Synthesizes film grain on top of `in` and stores the result to `out`. `out`
 // must already have been allocated and set to the same size and format as `in`.
 int ff_aom_apply_film_grain(AVFrame *out, const AVFrame *in,
                             const AVFilmGrainParams *params);
 
+// Parse AFGS1 parameter sets from an ITU-T T.35 payload. Returns 0 on success,
+// or a negative error code.
+int ff_aom_parse_film_grain_sets(AVFilmGrainAFGS1Params *s,
+                                 const uint8_t *payload, int payload_size);
+
+// Attach all valid film grain param sets to `frame`.
+int ff_aom_attach_film_grain_sets(const AVFilmGrainAFGS1Params *s, AVFrame *frame);
+
 #endif /* AVCODEC_AOM_FILM_GRAIN_H */