performance hack: exploit the fact that the x coordinates (diameters) of the hull...

performance hack: exploit the fact that the x coordinates (diameters) of the hull in DIRECT fall into only a few different values (although this may change in future modifications); also, we weren't handling the case of equal (x,y) points correctly in the hull code

darcs-hash:20070829052435-c8de0-4d2204875b3f788d8884aa57f55cc76a5907bace.gz