gradx   : horizontal gradient filter.
Computes the horizontal first derivative.
out-img = gradx(in-img) * (mul/1024) + add.
cmnd: gradx in-img,out-img, mul  ,  add
dflt: gradx    A  ,   B  , 1024  ,  128
range:       <A-D>,<A-D> ,<10000>,<+/-255>
