~1% faster overall on Conroe, mostly due to improved cache locality.
Also allows improved SIMD on some chroma functions (e.g. deblock).
This change also extends the API to allow direct NV12 input, which should be a bit faster than YV12.
This isn't currently used in the x264cli, as swscale does not have fast NV12 conversion routines, but it might be useful for other applications.

Note this patch disables the chroma SIMD code for PPC and ARM until new versions are written.