The RECTILINEAR nadir pass multiplied sampling coordinates by getRByRDist()
(distorted->ideal), but sampling positions must be moved to where the ideal
ray lands in the distorted image - the FORWARD distortion getRDistByR(),
same direction as GeometryCorrection.getImageCoordinates(). The old code
ADDED the common radial distortion to the output instead of removing it.

Evidence (graveyard ref 1763232420_274928, flat constant-disparity terrain
where translation flow must be uniform for a true pinhole image):
- plain NADIR-MERGED edge/center flow ratio 0.965 (expected 0.954 from the
  calibration polynomial: ~4.8 px barrel at the edge),
- old RECTILINEAR 0.952 - worse than uncorrected,
- COLMAP self-calibrated f=1003/k1=-0.385 on the old rectilinear stack.
Co-authored-by: Claude <claude@elphel.com>