(greymap-smooth-region greymap row col height width value)
Performs a smoothing operation on a given region of a greymap and returns that region. The region’s origin is row and col, its height and width are given in height and width respectively. value is the odd height of a square, wherefrom the average (mean) greymap-value is computed for each center pixel.
The smoothed region is returned as a new greymap.