NAME
       ReadPixel -- read the pen number value of the pixel at a
                    specified x,y location within a certain rastport.

   SYNOPSIS
       penno = ReadPixel( rp,    x,    y )
         d0               a1  d0:16 d1:16

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

   FUNCTION
       Combine the bits from each of the bit-planes used to describe
       a particular rastport into the pen number selector which that
       bit combination normally forms for the system hardware selection
       of pixel color.

   INPUTS
       rp -  pointer to a rastport structure
       (x,y) a point in the rastport

   RESULT
       penno - the pen number of the pixel at (x,y) is returned.
                -1 is returned if the pixel cannot be read for some reason.

   BUGS

   SEE ALSO
       writepixel()       graphics/rastport.h