FIX: Upstream Annex-B bitstream conversion fix for some BD mkv iso rips

ref https://github.com/FFmpeg/FFmpeg/commit/289b149cecb381522cc9ccdf382825330169c655#diff-0e332ff3db7b093bfee7dda7ed82ce19

Fix a quite bothersome playback issue with a number of, e.g, makemkv, 1-to-1 BD rips

/cc @davilla @t-nelson @jmarshallnz 
Probably. 
Having only a vague notion of what a NAL is and none of what an IDR picture is, I don't feel like updating the comment while upstream didn't, though...