The outcome of a butteraugli comparison.
score is the max-norm perceptual distance — the historical butteraugli
score, where < 1.0 is visually identical and > 2.0 is a visible
difference. pnorm_3 is libjxl's 3-norm aggregation of the difference map.
diffmap is nil unless the comparison ran with compute_diffmap: true, in
which case it is a packed, native-endian f32 binary of width * height
values (row-major, single channel) matching the input dimensions.