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