Added GrGLBlend.h|cpp with helper function AppendPorterDuffBlend() in preparation for SkComposeShad…

src/gpu/gl/GrGLBlend.cpp

Index: src/gpu/gl/GrGLBlend.cpp
diff --git a/src/gpu/gl/GrGLBlend.cpp b/src/gpu/gl/GrGLBlend.cpp
new file mode 100644
index 0000000000000000000000000000000000000000..0f73fbdbdbfb4c704aa65a5186bf1d9894a8bff2
--- /dev/null
+++ b/src/gpu/gl/GrGLBlend.cpp
@@ -0,0 +1,69 @@
+/*
+ * 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 "GrGLBlend.h"
+#include "gl/builders/GrGLFragmentShaderBuilder.h"
+
+static bool append_porterduff_term(GrGLFragmentBuilder* fsBuilder, SkXfermode::Coeff coeff,
+                                   const char* colorName, const char* srcColorName,
+                                   const char* dstColorName, bool hasPrevious) {
+    if (SkXfermode::kZero_Coeff == coeff) {
+        return hasPrevious;
+    } else {
+        if (hasPrevious) {
+            fsBuilder->codeAppend(" + ");
+        }
+        fsBuilder->codeAppendf("%s", colorName);
+        switch (coeff) {
+            case SkXfermode::kOne_Coeff:
+                break;
+            case SkXfermode::kSC_Coeff:
+                fsBuilder->codeAppendf(" * %s", srcColorName);
+                break;
+            case SkXfermode::kISC_Coeff:
+                fsBuilder->codeAppendf(" * (vec4(1.0) - %s)", srcColorName);
+                break;
+            case SkXfermode::kDC_Coeff:
+                fsBuilder->codeAppendf(" * %s", dstColorName);
+                break;
+            case SkXfermode::kIDC_Coeff:
+                fsBuilder->codeAppendf(" * (vec4(1.0) - %s)", dstColorName);
+                break;
+            case SkXfermode::kSA_Coeff:
+                fsBuilder->codeAppendf(" * %s.a", srcColorName);
+                break;
+            case SkXfermode::kISA_Coeff:
+                fsBuilder->codeAppendf(" * (1.0 - %s.a)", srcColorName);
+                break;
+            case SkXfermode::kDA_Coeff:
+                fsBuilder->codeAppendf(" * %s.a", dstColorName);
+                break;
+            case SkXfermode::kIDA_Coeff:
+                fsBuilder->codeAppendf(" * (1.0 - %s.a)", dstColorName);
+                break;
+            default:
+                SkFAIL("Unsupported Blend Coeff");
+        }
+        return true;
+    }
+}
+
+void GrGLBlend::AppendPorterDuffBlend(GrGLFragmentBuilder* fsBuilder, const char* srcColor,
+                                      const char* dstColor, const char* outColor,
+                                      SkXfermode::Mode mode) {
+
+    SkXfermode::Coeff srcCoeff, dstCoeff;
+    SkXfermode::ModeAsCoeff(mode, &srcCoeff, &dstCoeff);
+
+    fsBuilder->codeAppendf("%s =", outColor);
+    // append src blend
+    bool didAppend = append_porterduff_term(fsBuilder, srcCoeff, srcColor, srcColor, dstColor,
+                                            false);
+    // append dst blend
+    SkAssertResult(append_porterduff_term(fsBuilder, dstCoeff, dstColor, srcColor, dstColor,
+                                          didAppend));
+    fsBuilder->codeAppend(";");
+}