vf_psnr: sse2 optimizations for sum-squared-error.

The internal line accumulator for 16bit can overflow, so I changed that
from int to uint64_t in the C code. The matching assembly looks a little
weird but output looks correct.

(avx2 should be trivial to add later.)

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Reviewed-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
