Fix overflow in cdef distortion
Fix overflow in cdef distortion

With 12 bit input, an overflow can occur when squaring the sum of pixel
values over an 8x8 region.

  (8x8  x  (2^12-1))^2
= (2^6  x  (2^12-1))^2
= 2^12  x  (2^12-1)^2
= a 36-bit number

xiph/rav1e,xiph/rav1e