PERF: SLICImageFilter use faster TransformPhysicalPointToContinuousIndex

Replaced a call to `TransformPhysicalPointToContinuousIndex` with its faster
overload, within `SLICImageFilter::BeforeThreadedGenerateData()`.

Follow-up to pull request #2901
commit 6a8569e "PERF: Use the faster
`TransformPhysicalPointToContinuousIndex` overload"

Modules/Segmentation/SuperPixel/include/itkSLICImageFilter.hxx

@@ -160,8 +160,8 @@ SLICImageFilter<TInputImage, TOutputImage, TDistancePixel>::BeforeThreadedGenera
       const IndexType &                  idx = it.GetIndex();
       typename InputImageType::PointType pt;
       shrunkImage->TransformIndexToPhysicalPoint(idx, pt);
-      ContinuousIndexType cidx;
-      inputImage->TransformPhysicalPointToContinuousIndex(pt, cidx);
+      const ContinuousIndexType cidx =
+        inputImage->template TransformPhysicalPointToContinuousIndex<typename PointType::ValueType>(pt);
       for (unsigned int i = 0; i < ImageDimension; ++i)
       {
         cluster[numberOfComponents + i] = cidx[i];