Niklas Haas authored Oct 28, 2018 and Ronald S. Bultje committed Nov 19, 2018

The spec subtracts the signed offset from all of these when using them,
like it does for e.g. ar_coeffs_y_plus_128, although for some reason
the naming scheme is inconsistent here. Either way, it makes more sense
to treat them as signed integers than unsigned integers.

To avoid confusion since the name of the field is the same as the one in
the spec, we mark the type as int8_t (resp. int16_t for the 9-bit field)
to make it clear to the user that these are already signed integers.