NAME
        GetVPModeID -- get the 32 bit DisplayID from a viewport. (v36)

   SYNOPSIS
        modeID =  GetVPModeID( vp )
        d0                     a0

        ULONG GetVPModeID( struct viewport *);

   FUNCTION
        returns the normal display modeID, if one is currently  associated
        with this viewport.

   INPUTS
        vp -- pointer to a viewport structure.

   RESULT

        modeID -- a 32 bit DisplayInfoRecord identifier associated with
                  this viewport, or invalid_id.

   NOTES
        Test the return value of this function against INVALID_ID, not NULL.
        (INVALID_ID is defined in graphics/displayinfo.h).

   BUGS

   SEE ALSO
        graphics/displayinfo.h, modenotavailable()