Fix FL_MOUSEWHEEL event coordinates in subwindow (#210)

Issue #210: "Fl::event_x() & event_y() doesn't take into account
  embedded window's coords with Mouse wheel events"

In fact the calculation of Fl::event_x() and Fl::event_y() used the
wrong window coordinates if the event was sent to a subwindow.

src/Fl.cxx

@@ -1341,7 +1341,7 @@ int Fl::handle_(int e, Fl_Window* window)
       return 1;
     }
     // Finally try sending it to the window, the event occurred in
-    if (send_event(FL_MOUSEWHEEL, window, window)) return 1;
+    if (send_event(FL_MOUSEWHEEL, window, window->top_window())) return 1;
   default:
     break;
   }