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To improve decorrelation between components, we offset the dither matrix
slightly for each component. This is currently done by adding a hard-coded
offset of {0, 3, 2, 5} to each of the four components, respectively.
However, this represents a serious challenge when re-ordering SwsDitherOp
past a swizzle, or when splitting an SwsOpList into multiple sub-operations
(e.g. for decoupling luma from subsampled chroma when they are independent).
To fix this on a fundamental level, we have to keep track of the offset per
channel as part of the SwsDitherOp metadata, and respect those values at
runtime.
This commit merely adds the metadata; the update to the underlying backends
will come in a follow-up commit. The FATE change is merely due to the
added offsets in the op list print-out.