Offset AR coefficients to get the AV1 range

[andrey-norkin]

Shift AR parameters to get them to AV1 range. An alternative would be to not subtract 128 in the film grain synthesis algorithm, but this would make the implementation slightly different from AV1.

[haasn]

An alternative would be to not subtract 128 in the film grain synthesis algorithm, but this would make the implementation slightly different from AV1.

Strongly disapprove of deviating the synthesis implementation, so adding the +128 here seems like the best solution to me. Implementers in practice will know to skip the redundant addition/subtraction anyway.