The GPU compositor incorrectly extrapolates values of RGBA curves node.
That's because the code introduces a half-pixel offset to the color
values since they will be used to sample the curve maps. Those same
values are then used for extrapolation, which shouldn't take the
half-pixel value into account.
This patch fixes that by computing sampler coordinate in a separate
step.
Pull Request: https://projects.blender.org/blender/blender/pulls/116586
dc082f432a