Support AC-3 in Sega Dreamcast Sofdec files.

Fixes ticket #3184
Add an Opus and a Speex muxer.

Fixes ticket #3181.
avisynth: Move to av_new_packet/av_free_packet.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avisynth: make avs_library statically allocated.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
vp9: split x86 assembly in two files.

(And in future, loopfilter or intra pred could be put in their own
respective files also.)
vp9: minor refactorings in idct ssse3 assembly.

Make register usage in macros explicit; change mulsub_2w_4x to use 2
instead of 3 temp registers. Use this free'ed register to load 11585x2
once before the 2 1d idct4s instead of within each 1d idct4.
vp9: implement top/left half (4x4) sub-8x8-IDCT.

For that specific case (eob>3&&eob<=12), runtime of idct8x8 goes from
668 to 477 cycles. For all idct8x8, runtime goes from 521 to 490 cycles.