SkPDF: with opaque draws, treat SRC mode as SRC_OVER (cherrypick a681433 and 3098a75)

src/core/SkXfermodeInterpretation.h

Index: src/core/SkXfermodeInterpretation.h
diff --git a/src/core/SkXfermodeInterpretation.h b/src/core/SkXfermodeInterpretation.h
new file mode 100644
index 0000000000000000000000000000000000000000..d0a420f383d4ece6a1e77286af0e87c9d5449e8e
--- /dev/null
+++ b/src/core/SkXfermodeInterpretation.h
@@ -0,0 +1,30 @@
+/*
+ * 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 SkXfermodeInterpretation_DEFINED
+#define SkXfermodeInterpretation_DEFINED
+
+class SkPaint;
+
+/**
+ *  By analyzing the paint, we may decide we can take special
+ *  action. This enum lists our possible actions.
+ */
+enum SkXfermodeInterpretation {
+    kNormal_SkXfermodeInterpretation,      //< draw normally
+    kSrcOver_SkXfermodeInterpretation,     //< draw as if in srcover mode
+    kSkipDrawing_SkXfermodeInterpretation  //< draw nothing
+};
+
+/**
+ *  Given a paint, determine whether the paint's transfer mode can be
+ *  replaced with kSrcOver_Mode or not drawn at all.  This is used by
+ *  SkBlitter and SkPDFDevice.
+ */
+SkXfermodeInterpretation SkInterpretXfermode(const SkPaint&, bool dstIsOpaque);
+
+#endif  // SkXfermodeInterpretation_DEFINED