[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
That get_robust_point is introduced for the mapped_robust_point/robust_point compatibility.
But that will be removed at all, it did not work in all cases.
So I probably will remove the get_robust_point() function too.
In internal helper-structs we often access members directly.

Thanks for the fix, BTW.