Ensure Qt::WA_Mapped is set in case of obscured native windows.

Task-number: QTBUG-33520
Change-Id: I51f9b4634be29fd32f4ad9cc8b5d3e10b19ea2f5
Reviewed-by: Oliver Wolff <oliver.wolff@digia.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>