Remove SkLerpXfermode

src/effects/SkArithmeticMode.cpp

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkArithmeticMode.h"
 #include "SkColorPriv.h"
 #include "SkReadBuffer.h"
 #include "SkWriteBuffer.h"
 #include "SkString.h"
 #include "SkUnPreMultiply.h"
 #include "SkValue.h"
 #include "SkValueKeys.h"
 #if SK_SUPPORT_GPU
 #include "SkArithmeticMode_gpu.h"
 #endif
 
 static const bool gUseUnpremul = false;
 
 class SkArithmeticMode_scalar : public SkXfermode {
 public:
-    static SkArithmeticMode_scalar* Create(SkScalar k1, SkScalar k2, SkScalar k3, SkScalar k4,
-                                           bool enforcePMColor) {
+    static SkXfermode* Create(SkScalar k1, SkScalar k2, SkScalar k3, SkScalar k4,
+                              bool enforcePMColor) {
+        if (SkScalarNearlyZero(k1) && SkScalarNearlyEqual(k2, SK_Scalar1) &&
+            SkScalarNearlyZero(k3) && SkScalarNearlyZero(k4)) {
+            return SkXfermode::Create(SkXfermode::kSrc_Mode);
+        } else if (SkScalarNearlyZero(k1) && SkScalarNearlyZero(k2) && 
+                   SkScalarNearlyEqual(k3, SK_Scalar1) && SkScalarNearlyZero(k4)) {
+            return SkXfermode::Create(SkXfermode::kDst_Mode);
+        }
+
         return new SkArithmeticMode_scalar(k1, k2, k3, k4, enforcePMColor);
     }
 
     virtual void xfer32(SkPMColor dst[], const SkPMColor src[], int count,
                         const SkAlpha aa[]) const override;
 
     SK_TO_STRING_OVERRIDE()
     SK_DECLARE_PUBLIC_FLATTENABLE_DESERIALIZATION_PROCS(SkArithmeticMode_scalar)
 
 #if SK_SUPPORT_GPU
@@ skipping 235 matching lines @@
                                              fEnforcePMColor);
     }
     return true;
 }
 
 #endif
 
 SK_DEFINE_FLATTENABLE_REGISTRAR_GROUP_START(SkArithmeticMode)
     SK_DEFINE_FLATTENABLE_REGISTRAR_ENTRY(SkArithmeticMode_scalar)
 SK_DEFINE_FLATTENABLE_REGISTRAR_GROUP_END