Avoids allocations and therefore error checks: Syncing
hwaccel_picture_private across threads can't fail any more.
Also gets rid of an unnecessary pointer in structures and
in the parameter list of ff_hwaccel_frame_priv_alloc().
Reviewed-by: Anton Khirnov <anton@khirnov.net>
Reviewed-by: Lynne <dev@lynne.ee>
Tested-by: Lynne <dev@lynne.ee>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>