exor: exor-function of two bitplanes.
bitpl out = (bitpl 1 in) EXOR (bitpl 2 in).

cmd : exor bitpl 1 in,bitpl 2 in,bitpl out
dflt: exor     1     ,    2     ,    2
range:       <1,8>   ,  <1,8>   ,  <1,8>
