windowscodecs: Implement DllCanUnloadNow

[Vincent Povirk]

> Um, i thought that all goes through the factory...

That is incorrect in this case. Windowscodecs has several exports that
create COM objects. Even when they do go through the class factory,
the other COM objects can and usually do outlive the factory.

> So what do you think, should i go through interfaces or is a stub better for now?

A stub returning S_FALSE should be fine.