sti: add stubs for the IStillImageW interface (try 2)

[Henri Verbeet]

2009/9/30 Damjan Jovanovic <damjan.jov at gmail.com>:
> Changelog:
> * sti: add stubs for the IStillImageW interface (try 2)
>
This leaks, you need to return the internal IUnknown to the caller
somehow. I think an explicit QueryInterface for IUnknown is supposed
to return lpInternalUnkVtbl, and StiCreateInstanceW() probably as well
when the object is being aggregated.