CV Fisheye Undistort

Straightens a wide-angle image with the equidistant fisheye model (cv2.fisheye.initUndistortRectifyMap + remap). Feed camera_matrix and the FOUR-coefficient dist_coeffs from 'Fisheye Calibrate' - the five Brown-Conrady coefficients of the pinhole calibrator mean something different and produce a plausible but wrong result. 'new_camera' is the framing knob: 'same K' keeps the original focal length and throws away everything the straightening pushes off-frame, while 'estimate new K' calls estimateNewCameraMatrixForUndistortRectify, whose 'balance' scales the new focal length - 0 keeps the longest focal cv2 considers valid, 1 the shortest (measured on the shipped example: fx 166 against 50). Which of the two leaves black corners depends on the lens, so LOOK at both rather than assuming: on that example balance 0.0 leaves 3.7% of the frame black and 1.0 leaves 0.3%. Also outputs the K the result is expressed in, which is what any later projection needs. NOTE: cv2's estimator returns an all-NaN matrix when the calibration left k3/k4 unconstrained (which a fit can do while still reporting a fine RMS); this node detects that, keeps the original K and logs a warning - calibrate with CALIB_FIX_K3 | CALIB_FIX_K4 to get a usable estimate.