[QOL] Remember slider position for single layer mode in preview (#10758)

init

src/slic3r/GUI/IMSlider.hpp

@@ -179,6 +179,7 @@ private:
     int  m_max_value;
     int  m_lower_value;
     int  m_higher_value;
+    int  m_single_value; // ORCA
     bool m_dirty = false;
 
     bool m_render_as_disabled{ false };