Calculate horizontal (X) haar wavelet at the specified point.
The method calculates horizontal wavelet, which is a difference of two horizontally adjacent boxes' sums, i.e. A-B. A is the sum of rectangle with coordinates (x, y-radius, x+radius-1, y+radius-1). B is the sum of rectangle with coordinates (x-radius, y-radius, x-1, y+radiys-1).