[regression] QGraphicsScene::drawBackground() gets a QPainter with composition mode 'Source' if QGraphicsView::CacheBackground is enabled

When KDE trunk switched to Qt-4.6 we noticed that several of our games from kdegames stopped to draw background correctly.

We figured out this is due to a QGS::drawBackground() is getting a QPainter with composition mode set to 'Source' instead of the usual 'SourceOver'.
This happens only when view has cacheMode set to QGV::CacheBackground.

Having a QPainter in Source mode makes it impossible to properly draw a partly transparent content in drawBackground().

Parker came up with a simple example written in PyQt (attached) - i think you won't have any trouble converting it to C++ if needed