Encode multiple frames in prallel instead of dividing each frame into slices.
Improves speed, and reduces the bitrate penalty of threading.
It is no longer possible to re-encode a frame, so threaded scenecut detection
must run in the pre-me pass, which is faster but less precise.
It is now useful to use more threads than you have cpus. --threads=auto has
been updated to use cpus*1.5.
Minor changes to ratecontrol.
New options: --pre-scenecut, --mvrange-thread, --non-deterministic
git-svn-id: svn://svn.videolan.org/x264/trunk@607 df754926-b1dd-0310-bc7b-ec298dee348c