Do this by allocating AVBSFContext together with the data that is
currently in AVBSFInternal; or rather: Put AVBSFContext at the beginning
of a new structure called FFBSFContext (which encompasses more than just
the internal fields and is a proper context in its own right, hence the
name) and remove the AVBSFInternal altogether.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>