
  PG2DPROJ -- horizontal projections of a 2D surface

  Plot a colored projection a 2D surface Z(Y,X) array along the X or the Y
  direction.  The considered subset of Z is delimited in the first (Y)
  dimension by I1 and I2 and the second (X) by J1 and J2, inclusively.

                                  | backward view
                                  |
                                  |
                                  |
                                  v

                       [ . . . . . . . . . . ]
                       [ . . . . . . . . . . ]
   forward view ---->  [ . . . . . . . . . . ] <---- backward view
                       [ . . . . . . . . . . ]
                   Y   [ . . . . . . . . . . ]
                       [ . . . . . . . . . . ]
                       [ . . . . . . . . . . ]
                       [ . . . . . . . . . . ]

                                  ^    X
                                  |
                                  |
                                  |
                                  | forward view

  Hidden parts of the surface are managed by plotting the rows or the
  columns of Z with consecutive indices.  According to the order of
  indices (I1,I2) and (J1,J2), we may have a forward or a backward view.

