Core: Switch controller interface to render widget on booting

Previously, the Qt frontend would initialize the controller
interface on starting, resulting in the cursor position being
relative to the main window, instead of the render window.

Source/Core/DolphinQt/MainWindow.cpp

@@ -940,6 +940,11 @@ void MainWindow::HideRenderWidget(bool reinit)
       if (m_render_widget->isFullScreen())
         SetFullScreenResolution(focus);
     });
+
+    // The controller interface will still be registered to the old render widget, if the core
+    // has booted. Therefore, we should re-bind it to the main window for now. When the core
+    // is next started, it will be swapped back to the new render widget.
+    g_controller_interface.ChangeWindow(GetWindowSystemInfo(windowHandle()).render_surface);
   }
 }