It involves less allocations and therefore has the nice property
that deriving a reference from a reference can't fail.
This allows for considerable simplifications in
ff_h264_(ref|replace)_picture().
Switching to the RefStruct API also allows to make H264Picture
smaller, because some AVBufferRef* pointers could be removed
without replacement.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>