propag: extends objects within a mask.
A pixel of an extension layer is added if
the mask pixel is white.
cmd : propag # iter,bit,maskb,conn,edge,im
dflt: propag    1  , 1 ,  2  , 6  ,  0 , B
range:     <1,512><1,8><1,8><4,6,8><0,2><AD>