rv34: frame-level multi-threading

      Statistics for bourne.rmvb -an -f null

1 thread:  37.12s user 0.03s system  99% cpu 37.174 total
2 threads: 47.63s user 0.24s system 185% cpu 25.807 total
4 threads: 41.21s user 0.30s system 327% cpu 12.674 total