have canvas send discard instead of retain if the draw would overwrite everything

src/core/SkPaintPriv.h

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkPaintPriv_DEFINED
 #define SkPaintPriv_DEFINED
 
 #include "SkTypes.h"
 
 class SkBitmap;
 class SkImage;
 class SkPaint;
 
 class SkPaintPriv {
 public:
+    enum ShaderOverrideOpacity {
+        kNone_ShaderOverrideOpacity,        //!< there is no overriding shader (bitmap or image)
+        kOpaque_ShaderOverrideOpacity,      //!< the overriding shader is opaque
+        kNotOpaque_ShaderOverrideOpacity,   //!< the overriding shader may not be opaque
+    };
+    
     /**
      *  Returns true if drawing with this paint (or NULL) will ovewrite all affected pixels.
      *
      *  Note: returns conservative true, meaning it may return false even though the paint might
      *        in fact overwrite its pixels.
      */
-    static bool Overwrites(const SkPaint& paint);
+    static bool Overwrites(const SkPaint* paint, ShaderOverrideOpacity);
+
+    static bool Overwrites(const SkPaint& paint) {
+        return Overwrites(&paint, kNone_ShaderOverrideOpacity);
+    }
 
     /**
      *  Returns true if drawing this bitmap with this paint (or NULL) will ovewrite all affected
      *  pixels.
      */
     static bool Overwrites(const SkBitmap&, const SkPaint* paint);
 
     /**
      *  Returns true if drawing this image with this paint (or NULL) will ovewrite all affected
      *  pixels.
      */
     static bool Overwrites(const SkImage*, const SkPaint* paint);
 };
 
 #endif