Add generic get_optimized_sad implementation
Add generic get_optimized_sad implementation

NOTE: To force generic SAD implementation on devices supporting
vectorized variants, you now have to override both get_optimized_sad
and reg_sad to generic (only overriding get_optimized_sad on AVX2
hardware would just run all SAD blocks through reg_sad_avx2). Let's
see if there's a more sensible way to do it, but it's not trivial.

ultravideo/kvazaar,ultravideo/kvazaar,ultravideo/kvazaar,ultravideo/kvazaar,ultravideo/kvazaar