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.
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