SetWriteMode procedure

  DECLARATION:  SetWriteMode(WriteMode : Integer)

      PURPOSE:  Sets the write mode for line drawing

         UNIT:  Graph

      REMARKS:  The parameter, WriteMode can be one of the constants
                declared in the Graph Unit:

                          Const
                             CopyPut = 0;     { MOV }
                             XORPut  = 1;     { XOR }

                These constants correspond to a binary operation between
                each byte in the line and the corresponding byte on the screen.

                CopyPut uses the assembly language MOV instruction which
                places the line on the screen overwriting anything that was
                previously in the same place.

                XORPut uses the assembly language XOR instruction to combine
                the line with whatever is on the screen.  Two successive
                XORPut calls will erase the line and return the screen to
                its original state.

 RESTRICTIONS:   Must first be in graphics mode.