Fix sky not cleared when exiting race, would show up on kart selection screen

src/graphics/irr_driver.cpp

@@ -1576,6 +1576,8 @@ void IrrDriver::onUnloadWorld()
 {
     delete m_rtts;
     m_rtts = NULL;
+
+    suppressSkyBox();
 }
 // ----------------------------------------------------------------------------
 /** Sets the ambient light.

src/guiengine/widgets/model_view_widget.cpp

@@ -196,6 +196,8 @@ void ModelViewWidget::setupRTTScene(PtrVector<scene::IMesh, REF>& mesh,
     AlignedArray<Vec3>& mesh_scale,
     const std::vector<int>& model_frames)
 {
+    irr_driver->suppressSkyBox();
+
     if (m_rtt_main_node != NULL) m_rtt_main_node->remove();
     if (m_light != NULL) m_light->remove();
     if (m_camera != NULL) m_camera->remove();