lafv/matroska: add A_OPUS/EXPERIMENTAL codec name.

The exact packing of Opus inside Matroska is not finalized.
Use A_OPUS/EXPERIMENTAL as codec name, like mkvtoolnix.
The A_OPUS name stays to let ffmpeg open files it has produced
until now, but newly produced file use the EXPERIMENTAL version.
Once the spec is stabilized it will be possible to consider
options to ensure compatibility with these files.
mips: move vector_fmul_window_mips to libavutil

Reviewed-by: "Babic, Nedeljko" <nbabic@mips.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
mux: fix chunked interleaver

The code did not account properly for packets that where added to
the end of the packet list. Also flags for such packets where not
set correctly leading to incorrect chunked interleaving.

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