ENH: Add C++17 CTAD deduction guides to ImageScanlineIterator

Added class template argument deduction guides to `ImageScanlineIterator` and
`ImageScanlineConstIterator`, in order to support passing a `SmartPointer` as
constructor argument.

Author: N-Dekker

Modules/Core/Common/include/itkImageScanlineConstIterator.h

@@ -260,6 +260,12 @@ class ITK_TEMPLATE_EXPORT ImageScanlineConstIterator : public ImageConstIterator
   OffsetValueType m_SpanBeginOffset{}; // one pixel the beginning of the scanline
   OffsetValueType m_SpanEndOffset{};   // one pixel past the end of the scanline
 };
+
+// Deduction guide for class template argument deduction (CTAD).
+template <typename TImage>
+ImageScanlineConstIterator(SmartPointer<const TImage>, const typename TImage::RegionType &)
+  ->ImageScanlineConstIterator<TImage>;
+
 } // end namespace itk
 
 #ifndef ITK_MANUAL_INSTANTIATION

Modules/Core/Common/include/itkImageScanlineIterator.h

@@ -96,6 +96,11 @@ class ITK_TEMPLATE_EXPORT ImageScanlineIterator : public ImageScanlineConstItera
   Self &
   operator=(const ImageScanlineConstIterator<TImage> & it);
 };
+
+// Deduction guide for class template argument deduction (CTAD).
+template <typename TImage>
+ImageScanlineIterator(SmartPointer<TImage>, const typename TImage::RegionType &)->ImageScanlineIterator<TImage>;
+
 } // end namespace itk
 
 #ifndef ITK_MANUAL_INSTANTIATION