(Matt's description from yonks ago)
writing CBR essence with a KAG > 1 the filler is not
taken into account. There seem to be two ways to cope
(1) the byte count could by rounded up to > the
nearest KAG boundary (with a fudge for KAGs < 17 bytes)
(2) the required filler could also be indexed.
the second option is more proper, but not so easy.