Fix for https://github.com/libigl/libigl/issues/458

Former-commit-id: 0df658fbdc75a5012e45af80ff8f27afc3b327e7

include/igl/copyleft/cgal/minkowski_sum.cpp

@@ -106,6 +106,7 @@ IGL_INLINE void igl::copyleft::cgal::minkowski_sum(
     vJ[ee].resize(vG[ee].rows(),2);
     vJ[ee].col(0) = eJ.array() + (FA.rows()+1);
     vJ[ee].col(1).setConstant(ee);
+    offsets[ee+1] = offsets[ee] + vW[ee].rows();
   }
   // Combine meshes
   int n=0,m=0;
@@ -141,7 +142,7 @@ IGL_INLINE void igl::copyleft::cgal::minkowski_sum(
       W,
       G,
       SJ);
-    J = slice(DerivedJ(J),SJ,1);
+    slice(DerivedJ(J),SJ,1,J);
   }
 }