[algorithms][buffer] fix header for inclusion of disjoint

It is the point-box and not the box-box version that is needed; also the call to disjoint is not done via the free function but rather using the disjoint dispatch struct
Why not use `turn.get_robust_point()` instead of directly accessing the member variable?
Looking at the buffer's turn info class it seems to me that this should be the correct thing to do.
Same comment applies to the call to` geometry::expand` a few lines above.