STYLE: Avoid multiple GetIndex() calls in ContourExtractor2DImageFilter

Avoided calling `inputIt.GetIndex()` repetitively during one and the same
iteration. This is just a style improvement, as it is not a major performance
improvement. (`inputIt.GetIndex()` is likely to be very fast.)

Author: N-Dekker

Modules/Filtering/Path/include/itkContourExtractor2DImageFilter.hxx

@@ -353,11 +353,12 @@ ContourExtractor2DImageFilter<TInputImage>::GenerateDataForLabels()
     ImageRegionConstIteratorWithIndex<TInputImage> inputIt{ input, inputRegion };
     for (inputIt.GoToBegin(); !inputIt.IsAtEnd(); ++inputIt)
     {
+      const auto &      index = inputIt.GetIndex();
       BoundingBoxType & bbox = labelBoundingBoxes[inputIt.Get()];
-      bbox.min[0] = std::min(bbox.min[0], inputIt.GetIndex()[0]);
-      bbox.min[1] = std::min(bbox.min[1], inputIt.GetIndex()[1]);
-      bbox.max[0] = std::max(bbox.max[0], inputIt.GetIndex()[0]);
-      bbox.max[1] = std::max(bbox.max[1], inputIt.GetIndex()[1]);
+      bbox.min[0] = std::min(bbox.min[0], index[0]);
+      bbox.min[1] = std::min(bbox.min[1], index[1]);
+      bbox.max[0] = std::max(bbox.max[0], index[0]);
+      bbox.max[1] = std::max(bbox.max[1], index[1]);
     }
     // Build the extended regions from the bounding boxes
     for (const InputPixelType label : allLabels)