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

src/core/SkPaintPriv.cpp

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
-#include "SkPaintPriv.h"
-
 #include "SkBitmap.h"
 #include "SkColorFilter.h"
+#include "SkPaintPriv.h"
 #include "SkImage.h"
 #include "SkPaint.h"
 #include "SkShader.h"
 
-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
-};
-
 static bool changes_alpha(const SkPaint& paint) {
     SkColorFilter* cf = paint.getColorFilter();
     return cf && !(cf->getFlags() & SkColorFilter::kAlphaUnchanged_Flag);
 }
 
-static bool overwrites(const SkPaint* paint, ShaderOverrideOpacity overrideOpacity) {
+bool SkPaintPriv::Overwrites(const SkPaint* paint, ShaderOverrideOpacity overrideOpacity) {
     if (!paint) {
         // No paint means we default to SRC_OVER, so we overwrite iff our shader-override
         // is opaque, or we don't have one.
         return overrideOpacity != kNotOpaque_ShaderOverrideOpacity;
     }
 
     SkXfermode::SrcColorOpacity opacityType = SkXfermode::kUnknown_SrcColorOpacity;
 
     if (!changes_alpha(*paint)) {
         const unsigned paintAlpha = paint->getAlpha();
         if (0xff == paintAlpha && overrideOpacity != kNotOpaque_ShaderOverrideOpacity &&
             (!paint->getShader() || paint->getShader()->isOpaque()))
         {
             opacityType = SkXfermode::kOpaque_SrcColorOpacity;
         } else if (0 == paintAlpha) {
             if (overrideOpacity == kNone_ShaderOverrideOpacity && !paint->getShader()) {
                 opacityType = SkXfermode::kTransparentBlack_SrcColorOpacity;
             } else {
                 opacityType = SkXfermode::kTransparentAlpha_SrcColorOpacity;
             }
         }
     }
 
     return SkXfermode::IsOpaque(paint->getXfermode(), opacityType);
 }
 
-bool SkPaintPriv::Overwrites(const SkPaint& paint) {
-    return overwrites(&paint, kNone_ShaderOverrideOpacity);
-}
-
 bool SkPaintPriv::Overwrites(const SkBitmap& bitmap, const SkPaint* paint) {
-    return overwrites(paint, bitmap.isOpaque() ? kOpaque_ShaderOverrideOpacity
+    return Overwrites(paint, bitmap.isOpaque() ? kOpaque_ShaderOverrideOpacity
                                                : kNotOpaque_ShaderOverrideOpacity);
 }
 
 bool SkPaintPriv::Overwrites(const SkImage* image, const SkPaint* paint) {
-    return overwrites(paint, image->isOpaque() ? kOpaque_ShaderOverrideOpacity
+    return Overwrites(paint, image->isOpaque() ? kOpaque_ShaderOverrideOpacity
                                                : kNotOpaque_ShaderOverrideOpacity);
 }