gr_id	IN:	GR interface identifier returned by GRstart
	gr_ref	IN:	Reference number of the raster image

	Purpose	Maps the reference number of a raster image to an index.
	Return value	Returns the index of the image if successful and FAIL (or -1) otherwise.
	Description	GRreftoindex returns the index of the raster image specified by the parameter gr_ref.

	FORTRAN	integer function mgr2idx(gr_id, gr_ref)
		integer gr_id, gr_ref