Fixing build breakage.

This reverts commit b83eff7e.

To be used in the mouse callback like this:
    if (CV_EVENT_MOUSEWHEEL == CV_GET_MOUSEWHEEL_EVENT(event))
    {
       int delta= CV_GET_WHEEL_DELTA(event);
       // use delta...
    }

Small fix for extracting mouse coordinates on Windows, in a way that will always work with multiple monitors.
This is the way recommended by Microsoft.