ENH: Make default-constructors of RGBPixel and RGBAPixel constexpr

Tested by means of `CheckConstexprBeginAndEndOfContainer()`. Note that these
default-constructors were already `constexpr` _implicitly_, as implied by
`= default`, when `ITK_FUTURE_LEGACY_REMOVE` would be enabled.

Author: N-Dekker

Modules/Core/Common/include/itkRGBAPixel.h

@@ -81,7 +81,9 @@ class ITK_TEMPLATE_EXPORT RGBAPixel : public FixedArray<TComponent, 4>
 #ifdef ITK_FUTURE_LEGACY_REMOVE
   RGBAPixel() = default;
 #else
-  RGBAPixel() { this->Fill(0); }
+  constexpr RGBAPixel()
+    : Superclass()
+  {}
 #endif
 
   /** Pass-through constructor for the Array base class. */

Modules/Core/Common/include/itkRGBPixel.h

@@ -80,7 +80,9 @@ class ITK_TEMPLATE_EXPORT RGBPixel : public FixedArray<TComponent, 3>
 #ifdef ITK_FUTURE_LEGACY_REMOVE
   RGBPixel() = default;
 #else
-  RGBPixel() { this->Fill(0); }
+  constexpr RGBPixel()
+    : Superclass()
+  {}
 #endif
 
 #if defined(ITK_LEGACY_REMOVE)

Modules/Core/Common/test/itkRGBAPixelGTest.cxx

@@ -22,6 +22,11 @@
 #include <gtest/gtest.h>
 
 
+static_assert(itk::RangeGTestUtilities::CheckConstexprBeginAndEndOfContainer<itk::RGBAPixel<>>() &&
+              itk::RangeGTestUtilities::CheckConstexprBeginAndEndOfContainer<itk::RGBAPixel<std::uint8_t>>() &&
+              itk::RangeGTestUtilities::CheckConstexprBeginAndEndOfContainer<itk::RGBAPixel<float>>());
+
+
 // Tests that a RGBAPixel that is "value-initialized" (by empty braces, `{}`) is zero-filled.
 TEST(RGBAPixel, ValueInitializedIsZeroFilled)
 {

Modules/Core/Common/test/itkRGBPixelGTest.cxx

@@ -22,6 +22,11 @@
 #include <gtest/gtest.h>
 
 
+static_assert(itk::RangeGTestUtilities::CheckConstexprBeginAndEndOfContainer<itk::RGBPixel<>>() &&
+              itk::RangeGTestUtilities::CheckConstexprBeginAndEndOfContainer<itk::RGBPixel<std::uint8_t>>() &&
+              itk::RangeGTestUtilities::CheckConstexprBeginAndEndOfContainer<itk::RGBPixel<float>>());
+
+
 // Tests that a RGBPixel that is "value-initialized" (by empty braces, `{}`) is zero-filled.
 TEST(RGBPixel, ValueInitializedIsZeroFilled)
 {