Overlay: Resolve d3d9 hook TODO. Fix naming.

* Introduce commented constatns for vtable offsets of create methods.
* Fix variable naming
* Resolve TODO in HookCreateEx:
HardHook checks if it was already set up (baseaddr != NULL), and if it was
it ignores further setup calls. Hence, we can safely do a best-effort of
hooking create9 again.
We currently expect the IDirect3D9 and IDirect3D9Ex objects to be the
same object instance.
** If we wanted to support separate objects we would query both for
IUnknown to compare addresses, and potentially create yet another hook to
myCreateDevice for the create9 on d3d9ex.