Move cut-away stepper logic to mousedown handler

This makes scrolling work for the stepper widget, and overrides behaviour for scrolling anywhere else in the window.

src/openrct2/interface/Window.cpp

@@ -303,7 +303,7 @@ static bool window_other_wheel_input(rct_window* w, rct_widgetindex widgetIndex,
             return false;
         }
     }
-    else
+    else if (widgetType == WWT_STEPPER)
     {
         buttonWidgetIndex = wheel < 0 ? widgetIndex + 1 : widgetIndex + 2;
 
@@ -314,6 +314,10 @@ static bool window_other_wheel_input(rct_window* w, rct_widgetindex widgetIndex,
             return false;
         }
     }
+    else
+    {
+        return false;
+    }
 
     if (widget_is_disabled(w, buttonWidgetIndex))
     {