Mpi Grade Match

Composite a re-render over the original through a mask, graded to match it. Outside the mask both show the same content, so a per-channel gain + offset is fitted there per frame (content moving through the band is dropped from the fit) and applied to the source: the seam lines up and what the mask keeps keeps its own colours. Not MpiInpaintHeal, which moves a fill's colour toward its surroundings and would tint an added object; this one needs the original and the re-render of the same shot.