Add hidpi support for threshold selector and curves

rtgui/mydiagonalcurve.h

@@ -64,7 +64,7 @@ protected:
     bool bghistvalid;
     double locallabRef; // Locallab reference value to display in the background
 
-    void draw (int handle);
+    void updateDrawingArea (const int handle, const ::Cairo::RefPtr< Cairo::Context> &cr);
     void interpolate ();
     void findClosestPoint();
     CursorShape motionNotify(CursorShape type, double minDistanceX, double minDistanceY, int num);