Function:     GgiPlotXor

Purpose:      Obtain and set XOR plotting mode.

Category:     USER-INTERFACE, GRAPHICS

File:         ggi.src

Author:       J.P. Terlouw

Use:
              #include "ggi.h"
              bool oldmode, newmode;

              oldmode = GgiPlotXor(newmode);

                oldmode     -  current drawing mode.
                newmode     -  new drawing mode.

Description:  GgiPlotXor both obtains the current plotting mode and sets
              a new mode. XOR mode can be used to simulate non-destructive
              plotting.
              Using this mode, any part of a plot can be erased by simply
              drawing it again using the same colour as the first time.

Updates:      Mar  2, 1998: JPT, Document created.
              Jul  2, 1998: JPT, XOR mode maintained per plot window.