Add toSVGFEFuncR|G|B|AElement() fuctions, and use them.

Source/core/svg/SVGFEComponentTransferElement.cpp

 /*
  * Copyright (C) 2004, 2005, 2007 Nikolas Zimmermann <zimmermann@kde.org>
  * Copyright (C) 2004, 2005 Rob Buis <buis@kde.org>
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
@@ skipping 72 matching lines @@
     if (!input1)
         return 0;
 
     ComponentTransferFunction red;
     ComponentTransferFunction green;
     ComponentTransferFunction blue;
     ComponentTransferFunction alpha;
 
     for (Node* node = firstChild(); node; node = node->nextSibling()) {
         if (node->hasTagName(SVGNames::feFuncRTag))
-            red = static_cast<SVGFEFuncRElement*>(node)->transferFunction();
+            red = toSVGFEFuncRElement(node)->transferFunction();
         else if (node->hasTagName(SVGNames::feFuncGTag))
-            green = static_cast<SVGFEFuncGElement*>(node)->transferFunction();
+            green = toSVGFEFuncGElement(node)->transferFunction();
         else if (node->hasTagName(SVGNames::feFuncBTag))
-            blue = static_cast<SVGFEFuncBElement*>(node)->transferFunction();
+            blue = toSVGFEFuncBElement(node)->transferFunction();
         else if (node->hasTagName(SVGNames::feFuncATag))
-            alpha = static_cast<SVGFEFuncAElement*>(node)->transferFunction();
+            alpha = toSVGFEFuncAElement(node)->transferFunction();
     }
 
     RefPtr<FilterEffect> effect = FEComponentTransfer::create(filter, red, green, blue, alpha);
     effect->inputEffects().append(input1);
     return effect.release();
 }
 
 }