OSX specific, related to the wxWidgets 3.1.4: Fixed wrong UI-scaling

(wxWidgets 3.1.4 can binding of wxEVT_DPI_CHANGED now, so it was called msw_rescale() for most of the controls redundantly)

+ Fixed update of the DoubleSlider after changing of the system color

src/slic3r/GUI/GUI_Preview.cpp

@@ -410,6 +410,12 @@ void Preview::msw_rescale()
     refresh_print();
 }
 
+void Preview::sys_color_changed()
+{
+    if (m_layers_slider != nullptr)
+        m_layers_slider->sys_color_changed();
+}
+
 void Preview::jump_layers_slider(wxKeyEvent& evt)
 {
     if (m_layers_slider) m_layers_slider->OnChar(evt);