QPainter: nested QPainters causes a crash rather than a warning.

The current warning about using more than one paint device only works if painting is not redirected to another paint device. If painting is redirected, the presence of two painters is not detected and the application crashes. The patch (customer to attach) fixes this problem.

I've tested the patch against Qt 4.6.3 and using customer supplied test harness. I can confirm that it works as described and applications that attempt to use more than one paint device, where one of those is redirected will no longer crash but give a suitable warning in the debug output.

Test harness is attached.