This avoids allocations and error checks etc. as well
as duplicate pointer lists in the CodedBitstreamFooContexts.
It also avoids casting const away for use as opaque,
as the RefStruct API supports const opaques.

The fact that some of the units are not refcounted
(i.e. they are sometimes part of an encoding context
like VAAPIEncodeH264Context) meant that CodedBitstreamUnit
still contains two pointers, one to the content
and another ownership pointer, replacing the AVBufferRef* pointer.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>