use vector () (#2218) [ci skip]

include/igl/slice.cpp

@@ -147,7 +147,7 @@ IGL_INLINE void igl::slice(
   {
     for (int j = 0; j < yn; j++)
     {
-      Y(i, j) = X(R(i, 0), C(j, 0));
+      Y(i, j) = X(R(i), C(j));
     }
   }
 }