CloseGraph procedure

  DECLARATION:  CloseGraph;

      PURPOSE:  Closes down the graphics system

         UNIT:  Graph

      REMARKS:  Returns the system to the original screen mode detected
                prior to entering graphics mode and frees the memory
                allocated on the heap for the graphics scan buffer.
                This also deallocates driver and font memory buffers if they
                were allocated by calls to GraphGetMem and GraphFreeMem.

 RESTRICTIONS:  Must first be in graphics mode.