deband

Smooths banding the SOURCE carries, which dithering cannot touch because it is already in the decoded samples.

Works like mpv's: sample a small ring around each texel, and where the neighbourhood is flat enough to be a band rather than an edge, replace the sample with its average and add a little grain. Costs real texture taps, so it is the pass to measure before defaulting on.

Pairs with dither, and the pairing is not a suggestion: into an 8-bit target this pass can only redistribute a hard step into a mixed transition, because there is no value between two adjacent 8-bit levels for the smoothed result to land on. RenderQuality.Standard turns both on for that reason.