Refactor 4f gradients using trait templates

src/effects/gradients/Sk4fGradientBase.h

 /*
  * Copyright 2016 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef Sk4fGradientBase_DEFINED
 #define Sk4fGradientBase_DEFINED
 
@@ skipping 41 matching lines @@
     uint8_t                      fFlags;
     bool                         fDither;
     bool                         fColorsArePremul;
 
 private:
     using INHERITED = SkShader::Context;
 
     void addMirrorIntervals(const SkGradientShaderBase&,
                             const Sk4f& componentScale, bool reverse);
 
-    template<typename DstType, SkShader::TileMode tileMode>
+    template<DstType, SkShader::TileMode tileMode>
     class TSampler;
 
-    template <typename DstType, ApplyPremul>
-    void shadePremulSpan(int x, int y, DstType[], int count) const;
+    template <DstType dstType, ApplyPremul premul>
+    void shadePremulSpan(int x, int y, typename DstTraits<dstType, premul>::Type[],
+                         int count) const;
 
-    template <typename DstType, ApplyPremul, SkShader::TileMode tileMode>
-    void shadeSpanInternal(int x, int y, DstType[], int count) const;
+    template <DstType dstType, ApplyPremul premul, SkShader::TileMode tileMode>
+    void shadeSpanInternal(int x, int y, typename DstTraits<dstType, premul>::Type[],
+                           int count) const;
 };
 
 #endif // Sk4fGradientBase_DEFINED