This was a regression that came in when I switched to using the
h.264 annex b filter all the time. As the filter modifies extradata,
its use violates the statelessness assumption that exists in the
'ffmpeg' command line tool, and maybe elsewhere. It assumes that
a docoder can be reinitalised and pointed to an existing stream and
get the same results.

For now, the only way to meet this requirement is to backup the
extradata.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>