vp8: frame-multithreading.

Tested on a Mac Pro, 2 CPUs, 2 cores each, OSX 10.6.6:

time ./ffmpeg -v 0 -vsync 0 -threads [1234] -i \
  ~/Downloads/sintel_trailer_1080p_vp8_vorbis.webm \
  -f null -vcodec rawvideo -an -
1: 0m14.630s (89.9 fps)
2: 0m8.056s (163.2 fps)
3: 0m5.882s (223.6 fps)
4: 0m4.952s (265.6 fps)

time ./ffmpeg -v 0 -vsync 0 -threads [1234] -i \
  ~/Downloads/Elephants_Dream-720p-Stereo.webm \
  -f null -vcodec rawvideo -an -
1: 1m12.962s (215.1 fps)
2: 0m44.682s (351.2 fps)
3: 0m31.183s (503.2 fps)
4: 0m25.284s (620.6 fps)

Signed-off-by: Anton Khirnov <anton@khirnov.net>