[x264-devel] optimized variance calculation

[David Pethes]

Hi,
this patch adds dedicated function for variance calculation.
It saves roughly 60 cycles on K8 compared to sad+ssd combination. 
Overall speedup is quite negligible, because it's rarely used - about 
0.1 fps improvement with fast settings (tested with --crf 20 -A none 
--aq-mode 2).
Regards,

David Pethes
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