use ternary operator

dgl/src/ImageWidgets.cpp

@@ -522,9 +522,7 @@ bool ImageKnob::onMouse(const MouseEvent& ev)
     if (ev.button > 3 || ev.button < 1)
         return false;
 
-    if (ev.button == 3)
-        fRightClicked = true;
-    else fRightClicked = false;
+    fRightClicked = (ev.button == 3) ? true : false;
 
     if (ev.press)
     {