win32u: Try the next top-level window if hit-testing the result window returns HTTRANSPARENT.
Fix clicking Toad for Oracle treeview nodes doesn't function. The left button click is sent to a top-level TSafeHintWindow, which returns HTTRANSPARENT when handling WM_NCHITTEST. Tests show that in such cases, the mouse message should be sent to the next top-level window in the z-order.