d3dx: VertexBuffer should keep reference to the parent device object.

[H. Verbeet]

> ChangeLog:
> d3dx: VertexBuffer should keep reference to the parent device object.

That's probably the best solution for the moment.
>+    /* Parent reference */
>+    LPDIRECT3DDEVICE9       parentDevice;
However, the name parentDevice is somewhat misleading, in particular
considering the meaning of "parent" in wined3d. From the point of view
of the d3d8/d3d9 object it's simply the device.