Added function CreateFrom2Procs() in new namespace GrXfermodeFragmentProcessor

src/gpu/effects/GrXfermodeFragmentProcessor.cpp

+/*
+ * Copyright 2015 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "effects/GrXfermodeFragmentProcessor.h"
+
+#include "GrFragmentProcessor.h"
+#include "SkComposeShader.h"
+#include "effects/GrConstColorProcessor.h"
+
+const GrFragmentProcessor* GrXfermodeFragmentProcessor::CreateFrom2Procs(
+        const GrFragmentProcessor* src, const GrFragmentProcessor* dst, SkXfermode::Mode mode) {
+    if (SkXfermode::kLastCoeffMode < mode) {
+        return nullptr;
+    }
+    switch (mode) {
+        case SkXfermode::kClear_Mode:
+            return GrConstColorProcessor::Create(GrColor_TRANS_BLACK,
+                                                 GrConstColorProcessor::kIgnore_InputMode);
+            break;
+        case SkXfermode::kSrc_Mode:
+            return SkRef(src);
+            break;
+        case SkXfermode::kDst_Mode:
+            return SkRef(dst);
+            break;
+        default:
+            return new GrComposeEffect(dst, src, mode);
+    }
+}