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

src/gpu/gl/GrGLBlend.h

Index: src/gpu/gl/GrGLBlend.h
diff --git a/src/gpu/gl/GrGLBlend.h b/src/gpu/gl/GrGLBlend.h
new file mode 100644
index 0000000000000000000000000000000000000000..af447b47bf29704276e07b88779d70cecf944b5f
--- /dev/null
+++ b/src/gpu/gl/GrGLBlend.h
@@ -0,0 +1,24 @@
+/*
+ * Copyright 2015 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef GrGLBlend_DEFINED
+#define GrGLBlend_DEFINED
+
+#include "SkXfermode.h"
+
+class GrGLFragmentBuilder;
+
+namespace GrGLBlend {
+    /*
+     * Appends GLSL code to fsBuilder that assigns a specified blend of the srcColor and dstColor
+     * variables to the outColor variable.
+     */
+    void AppendPorterDuffBlend(GrGLFragmentBuilder* fsBuilder, const char* srcColor,
+                               const char* dstColor, const char* outColor, SkXfermode::Mode mode);
+};
+
+#endif