This fixes the tests filter-refcmp-ssim-yuv and filter-refcmp-ssim-rgb
on i386 after breaking in fcc0424c.
Signed-off-by: NMartin Storsjö <martin@martin.st>

Use doubles for accumulating floats.

~20% faster than ssse3. Also enabled for x86_32
Reviewed-by: NRonald S. Bultje <rsbultje@gmail.com>
Signed-off-by: NJames Almer <jamrial@gmail.com>

Reviewed-by: NMichael Niedermayer <michaelni@gmx.at>
Signed-off-by: NJames Almer <jamrial@gmail.com>

Both are 2-2.5x faster than their C counterpart.
Reviewed-by: NPaul B Mahol <onemda@gmail.com>
Reviewed-by: NJames Almer <jamrial@gmail.com>
Signed-off-by: NMichael Niedermayer <michael@niedermayer.cc>