Layered (translucent/non-opaque) windows do not receive WM_PAINT,
expose events need to be generated. Improve
6800728d09 to handle transient
children as well.
Task-number: QTBUG-17548
Change-Id: Id113604512692dfbea1f2b10d0db3068213cf599
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>