NAME
       SetWriteMask -- Set the pixel write mask value for a rastport (v39).


   SYNOPSIS
       success=SetWriteMask ( rp, msk )
          d0                   a0   d0

        ULONG SetWriteMask(struct rastport *,ulong)

   FUNCTION
        Set the current value of the bit write mask for the rastport.
        bits of the pixel with zeros in their mask will not be modified by
        subsequent drawing operations.

   INPUTS
       rp  =  a pointer to a valid rastport structure.
        msk =  a longword mask value.

        Graphics devices which do not support per-bit masking will
        return 0 (failure).
   BUGS

   NOTES

   SEE ALSO
        graphics/gfxmacros.h