wined3d: universal surface convertor function for unsigned integer color formats(2nd attempt)

[Stefan Dösinger]

> It has been a couple of days since I've submitted patch. Patch still
> isn't in git. Will it be accepted? If not, what should be changed to
> make it "acceptable"?
I think the problem was that I forgot to reply to your mail.

I think the idea of using this as a fallback if no special conversion function is needed is ok.