http://bugs.winehq.org/show_bug.cgi?id=23946
--- Comment #9 from Vincent Povirk madewokherd@gmail.com 2010-08-16 15:49:18 --- And here's the explanation: http://www.winehq.org/pipermail/wine-patches/2010-August/092067.html
This should be much simpler to implement than proper RGBA rendering.