Child widgets do not receive paint events under certain cirumstances

When a child widget with WA_StaticContents and WA_OpaquePaintEvent set is spontaneously exposed due to a parent resize, the child will not receive a paint event, and the backing store for the child area will remain black.

The attached minimal example demonstrates the issue.

I believe the solution is due to a subtle logic error introduced into the new widget backing store code in Qt5. I will submit a change set shortly.