After quite a lot debugging I was able to find the cause of the swing artifacts.
The problem was that render also honors the clip-mask when a pixmap is used as source, of course this completly messed up the double buffering Repaint-Manager.
Now swing application look almost like they should:
The last remaining piece to make common swing apps useable is copyArea, I hope I get it done in the next few days. There are also other annoying and serious bugs, but this one will make swing apps just work :)
After the most serious bugs I know about are fixed, I hope I can upload that "hack"-release of my pipeline to my OpenJDK repository, and start working on the final design.