vp3: Frame-based multithreading support

      Decode times for big_buck_bunny_720p_stereo:

1 thread:
real    1m14.227s
user    1m13.104s
sys     0m1.108s

2 threads: (33% faster)
real    0m49.329s
user    1m33.735s
sys     0m1.834s

3 threads: (44% faster)
real    0m41.593s
user    1m44.884s
sys     0m1.967s