gradx   : horizontal gradient filter.
Calculates the horizontal 1st directive.
out img = gradx(in img) * (mul/1024) + add.
cmnd: gradx in img,out img, mul  ,  add
dflt: gradx    A  ,   B  , 1024  ,  128
range:      <A,B> ,<A,B> ,<10000>,<+/-255>
