Relocate the specialized linear gradient interval builder

src/effects/gradients/Sk4fLinearGradient.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 Sk4fLinearGradient_DEFINED
 #define Sk4fLinearGradient_DEFINED
 
 #include "Sk4fGradientBase.h"
 #include "SkLinearGradient.h"
 
 class SkLinearGradient::
 LinearGradient4fContext : public GradientShaderBase4fContext {
 public:
     LinearGradient4fContext(const SkLinearGradient&, const ContextRec&);
 
     void shadeSpan(int x, int y, SkPMColor dst[], int count) override;
     void shadeSpan4f(int x, int y, SkPM4f dst[], int count) override;
 
 private:
     using INHERITED = GradientShaderBase4fContext;
 
+    void addMirrorIntervals(const SkLinearGradient&, const Sk4f& componentScale,
+                            bool dx_is_pos);
+
     template<typename DstType, TileMode>
     class LinearIntervalProcessor;
 
     template <typename DstType, bool premul>
     void shadePremulSpan(int x, int y, DstType[], int count) const;
 
     template <typename DstType, bool premul, SkShader::TileMode tileMode>
     void shadePremulTileSpan(int x, int y, DstType[], int count) const;
 
     template <typename DstType, bool premul, SkShader::TileMode tileMode>
     void shadeSpanInternal(int x, int y, DstType[], int count) const;
+
+    const Interval* findInterval(SkScalar fx) const;
+
+    mutable const Interval*      fCachedInterval;
 };
 
 #endif // Sk4fLinearGradient_DEFINED