Add nadir_undistort (boolean, default true) to IntersceneMatchParameters
at all 6 required locations (field, dialog, read, save, load, clone).

When enabled, renderNadirSequence() undistorts the pX,pY coordinates of
each scene's nadir disparity map before calling renderGPUFromDSI(), so the
rendered NADIR-MERGED TIFF images are rectilinear (pinhole projection)
rather than retaining the sensor's native radial distortion.

Analysis of the Boson 640 calibration (lwir16-06-72-00.calib-tiff) showed:
- True distortion polynomial terms: < 1 px magnitude across the full image
- The dominant issue was d = K0 = 1.00997 (a ~1% focal-length constant offset)
  which, when modeled as PINHOLE with the nominal focal length, curves the
  ground plane into the 'Little Prince planet' sphere effect seen in Blender
- With nadir_undistort=true, COLMAP can use a pure PINHOLE model with the
  correct effective focal length (f * d = 1121.8 px instead of 1110.8 px)

The undistortPxPy() helper mirrors getWorldCoordinates() in GeometryCorrection:
  rD_mm = ||(pX-cx, pY-cy)|| * pixelSize_mm
  factor = getRByRDist(rD_mm / distortionRadius)
  pX_rect = (pX - cx) * factor + cx
Co-authored-by: Claude <claude@elphel.com>