The SEI message code uses the AVBuffer API for its SEI messages
and contained buffers (like the extension buffer for HEVC
or the user data (un)registered payload buffers).

Contrary to the ordinary CBS code (where some of these
contained buffer references are actually references
to the provided AVPacket's data so that one can't replace
them with the RefStruct API), the CBS SEI code never uses
outside buffers at all and can therefore be switched entirely
to the RefStruct API. This avoids the overhead inherent
in the AVBuffer API (namely the separate allocations etc.).

Notice that the refcounting here is actually currently unused;
the refcounts are always one (or zero in case of no refcounting);
its only advantage is the flexibility provided by custom
free functions.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>