DX6 more compatibility.

[Stefan Dösinger]

>
> Good summary about C++ templates and Co skipped
>

> And this is where I discovered
> what a thunk it: Exactly a function that adjusts the this pointer
> before
> calling the main implementation; these thunks are unavoidable if
> functions from one vtable are forwarded to functions belonging to
> another vtable implemented by the same object.
That's not necessarily the case, I think it is not legal to compare the
vtable pointers, so they should not matter. If a vtable inherits from the
other, using one vtable is ok. Here we thought IDDS7  inherits from IDSS4.
While it looks like that in the interface, the behavior is different, so the
inheritance relation isn't given. (Just to make it clear)