gdiplus: Move the burden of supporting GIF encoding to the WIC backend.

[Vincent Povirk]

I don't have a strong opinion on this, but committing this before we
have support in WIC seems a bit sideways? What we were doing before
was broken, but this just makes it broken in a different way. Before,
we weren't reporting all formats native does, but now we're reporting
formats we can't encode.

So, this is just going to break or fix applications at random?