sparse_disp : add block write code

Grid the disparities 1024 output pixels at a time.  This helps fix the huge memory allocation for sparse_disp in its last stages, and avoids some hangups that can happen when griddata has to search an enormous triangulation for the grid points.
The blockwise gridding respects the hole-filling radius established in d621f1afb12bc8725fd9f8bb837d5421a819019d