Fill the background pixmap with black after creating it.
QPixmap documentation says explicitly that the pixmap is not initialized after construction, and themes don't necessarily fill the contents of all screens.

For example the opensuse branded theme leaves one of my screens completely alone.
Together with the newest nvidia driver, this creates the ugliest login screen possible (one screen filled with complete garbage).

Please review!
Eventually this is a candidate for 4.2 backporting?