Restore rescaling opacity to B&C editor

This was originally removed in d031416, since I originally thought it
did not make sense for a B&C editor to also modify opacity.

Users are actually expecting and relying on this, however, and adjusting
the opacity to hide the voxels below the minimum is necessary for thresholding
them out. Thus, we are restoring this behavior.

Signed-off-by: Patrick Avery <patrick.avery@kitware.com>

Author: psavery

tomviz/BrightnessContrastWidget.cxx

@@ -318,6 +318,7 @@ class BrightnessContrastWidget::Internals
   void rescaleNodes(double newMin, double newMax)
   {
     tomviz::rescaleNodes(m_uncroppedLut, newMin, newMax);
+    tomviz::rescaleNodes(m_uncroppedOpacity, newMin, newMax);
     pushChanges();
     updateGui();
   }