[Windows] Wheel events are redirected to translucent regions of translucent windows

Scenario

• Opaque top level widget
• Translucent window in front of it

To reproduce:

• Move mouse wheel over translucent region of translucent window
• Expected: Opaque top level gets wheel events
• Actual: translucent window gets wheel events

The case is described in this StackOverflow question. To quote:

Mouse clicks work as expected:

• fall through transparent regions to the opaque widget;
• get caught by transparent widget when clicked in its opaque areas.

Wheel events always get caught by the transparent widget no matter where they occur.

The same project does work correctly with Qt4.8.