avcodec/aom_film_grain: add AOM film grain synthesis

Implementation copied wholesale from dav1d, sans SIMD, under permissive
license. This implementation was extensively verified to be bit-exact,
so it serves as a much better starting point than trying to re-engineer
this from scratch for no reason. (I also authored the original
implementation in dav1d, so any "clean room" implementation would end up
looking much the same, anyway)

The notable changes I had to make while adapting this from the dav1d
code-base to the FFmpeg codebase include:

- reordering variable declarations to avoid triggering warnings
- replacing several inline helpers by avutil equivalents
- changing code that accesses frame metadata
- replacing raw plane copying logic by av_image_copy_plane

Apart from this, the implementation is basically unmodified.

libavcodec/aom_film_grain.h

+/*
+ * AOM film grain synthesis
+ * Copyright (c) 2021 Niklas Haas <ffmpeg@haasn.xyz>
+ *
+ * This file is part of FFmpeg.
+ *
+ * FFmpeg is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * FFmpeg is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with FFmpeg; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+/**
+ * @file
+ * AOM film grain synthesis.
+ * @author Niklas Haas <ffmpeg@haasn.xyz>
+ */
+
+#ifndef AVCODEC_AOM_FILM_GRAIN_H
+#define AVCODEC_AOM_FILM_GRAIN_H
+
+#include "libavutil/film_grain_params.h"
+
+// 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);
+
+#endif /* AVCODEC_AOM_FILM_GRAIN_H */