NAME
       WritePixel -- Change the pen num of one specific pixel in a
                     specified rastport.

   SYNOPSIS
       error = WritePixel(  rp, x,  y)
         d0                 a1 D0  D1

        long writepixel( struct rastport *, short, short );

   FUNCTION
       Changes the pen number of the selected pixel in the specified
       rastport to that currently specified by pena, the primary
       drawing pen. Obeys minterms in rastport.

   INPUTS
       rp - a pointer to the rastport structure
       (x,y) - point within the rastport at which the selected
           pixel is located.

   RESULT
       error = 0 if pixel succesfully changed
              = -1 if (x,y) is outside the rastport

   BUGS

   SEE ALSO
       readpixel() graphics/rastport.h