do not add nodes to curves when clicking on the preview window unless ctrl is clicked

Candidate fix for #3862

rtgui/myflatcurve.cc

@@ -1278,6 +1278,10 @@ void MyFlatCurve::pipetteMouseOver (CurveEditor *ce, EditDataProvider *provider,
 // returns true if a point is being dragged
 bool MyFlatCurve::pipetteButton1Pressed(EditDataProvider *provider, int modifierKey)
 {
+    if (!(modifierKey & GDK_CONTROL_MASK)) {
+        return false;
+    }
+    
     if (edited_point > -1) {
         return false;
     }