Fix Sonarcube bug : use of const ref

This fixes one bug reported by Sonarcube

src/render/opengl/Render_opengl.cpp

@@ -715,7 +715,7 @@ void Render_opengl::draw_text_value(long int id, double text, double y) {
     }
 }
 
-void Render_opengl::draw_text(const Element_pos x, const Element_pos y, const Element_pos size, const std::string s) {
+void Render_opengl::draw_text(const Element_pos x, const Element_pos y, const Element_pos size, const std::string &s) {
     if (_draw_ruler)
         return; /* do not draw text for ruler */
     if (_draw_state)