windowscodecsext: Implement WICCreateColorTransform_Proxy and IWICColorTransform_Initialize_Proxy.

[Vincent Povirk]

OK, I'm about to say something weird, and I'll understand if everyone
else decides to ignore it.

I don't think WICCreateColorTransform_Proxy, or any of the Proxy
functions, should initialize or make use of COM.

Instead, I think it should work by calling WICCreateImagingFactory_Proxy.

I suspect that part of the purpose of the _Proxy functions is to make
it possible to use WIC without needing COM at all, which is something
we may want in the future if we want to use it from user32.

(Another consequence of this is that we should be able to create
instances of builtin decoders/encoders without COM, which we currently
can't do. But I'd have to write a test for that, and currently there's
been no demonstrated need for it.)