libdav1d: use film grain export flag to export AVFilmGrainParams side data

libdav1d: use film grain export flag to export AVFilmGrainParams side data

This patch is relatively straightforward with one exception:
the decoder option flag.
The option was introduced to troubleshoot but its existence is conflicting
and redundant now that we have a codec-generic flag.
Hence this patch deprecates it.

The way it interacts with AV_CODEC_EXPORT_DATA_FILM_GRAIN is as follows:

If filmgrain is unset and AV_CODEC_EXPORT_DATA_FILM_GRAIN is
present, disable film grain application and export side data.

If filmgrain is set to 0, disable film grain and export side data.

If filmgrain is set to 1, apply film grain but export side data if
the AV_CODEC_EXPORT_DATA_FILM_GRAIN flag is set. This may result in
double film grain application, but the user has requested it by setting
both.