Add an option on SkImageDecoder to skip writing 0s.

include/core/SkImageDecoder.h

Index: include/core/SkImageDecoder.h
diff --git a/include/core/SkImageDecoder.h b/include/core/SkImageDecoder.h
index a8cbe6341332e4efcffe751589736c7b67d01758..16216c8ccbce70b5894d88b2bf39d56a511d3dd0 100644
--- a/include/core/SkImageDecoder.h
+++ b/include/core/SkImageDecoder.h
@@ -59,6 +59,23 @@ public:
     */
     const char* getFormatName() const;
 
+    /** Whether the decoder should skip writing zeroes to output if possible.
+    */
+    bool getSkipWritingZeroes() const { return fSkipWritingZeroes; }
+
+    /** Set to true if the decoder should skip writing any zeroes when
+        creating the output image.
+        This is a hint that may not be respected by the decoder.
+        It should only be used if it is known that the memory to write
+        to has already been set to 0; otherwise the resulting image will
+        have garbage.
+        This is ideal for images that contain a lot of completely transparent
+        pixels, but may be a performance hit for an image that has only a
+        few transparent pixels.
+        The default is false.
+    */
+    void setSkipWritingZeroes(bool skip) { fSkipWritingZeroes = skip; }
+
     /** Returns true if the decoder should try to dither the resulting image.
         The default setting is true.
     */
@@ -507,6 +524,7 @@ private:
     PrefConfigTable         fPrefTable;     // use if fUsePrefTable is true
     bool                    fDitherImage;
     bool                    fUsePrefTable;
+    bool                    fSkipWritingZeroes;
     mutable bool            fShouldCancelDecode;
     bool                    fPreferQualityOverSpeed;
     bool                    fRequireUnpremultipliedColors;