avcodec/h264_slice: compute and export film grain seed

From SMPTE RDD 5-2006, the grain seed is to be computed from the
following definition of `pic_offset`:

> When decoding H.264 | MPEG-4 AVC bitstreams, pic_offset is defined as
> follows:
>   - pic_offset = PicOrderCnt(CurrPic) + (PicOrderCnt_offset << 5)
> where:
>   - PicOrderCnt(CurrPic) is the picture order count of the current frame,
>     which shall be derived from [the video stream].
>
>   - PicOrderCnt_offset is set to idr_pic_id on IDR frames. idr_pic_id
>     shall be read from the slice header of [the video stream]. On non-IDR I
>     frames, PicOrderCnt_offset is set to 0. A frame shall be classified as I
>     frame when all its slices are I slices, which may be optionally
>     designated by setting primary_pic_type to 0 in the access delimiter NAL
>     unit. Otherwise, PicOrderCnt_offset it not changed. PicOrderCnt_offset is
>     updated in decoding order.

Co-authored-by: James Almer <jamrial@gmail.com>
Signed-off-by: Niklas Haas <git@haasn.dev>
Signed-off-by: James Almer <jamrial@gmail.com>

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