Opacity function doesn't support log mapping.

Since piecewise funciton doesn't log mapping, vtkCompositeControlPointsItem
cannot support showing log-space when in opacity-function mode.

Change-Id: Ib74dddcd347fd49aa2f6bc36cdc9a3daedf6f76c

Charts/Core/vtkCompositeControlPointsItem.cxx

@@ -157,7 +157,8 @@ void vtkCompositeControlPointsItem::SetColorTransferFunction(vtkColorTransferFun
 //-----------------------------------------------------------------------------
 bool vtkCompositeControlPointsItem::UsingLogScale()
 {
-  return (this->ColorTransferFunction &&
+  return (this->PointsFunction != OpacityPointsFunction &&
+    this->ColorTransferFunction &&
     this->ColorTransferFunction->UsingLogScale());
 }