New methods in OpticalFlow.java:
- getNadirDisparityMaps(): transforms SfM terrain disparity from the
  segment reference into each scene's own camera frame using
  Cuas.transformFromVirtual(). Returns double[][][nscene][ntiles][3].
- renderNadirSequence(): renders each scene from its own position
  (scene_xyz/atr=ZERO3, ref_scene=scene itself) using per-scene
  pXpYD from getNadirDisparityMaps(). ERS rolling-shutter velocity
  corrections are still applied. Output: <ts>-NADIR-MERGED.tiff.

Unlike renderSceneSequence() (TERRAIN-DISP), which warps all frames
to the reference for zero ground parallax, renderNadirSequence() produces
natural-parallax perspective images from each scene's own viewpoint —
the correct input format for OpenMVS/COLMAP dense reconstruction.

New parameter in IntersceneMatchParameters:
- nadir_gen (boolean, default false): checkbox under 'Flat ground' tab,
  before fgnd_gen_optho. Enables nadir rendering in the existing
  TERRAIN-DISP pipeline block.
Co-authored-by: Claude <claude@elphel.com>