change pixel-serializer to support reencoding existing data

include/core/SkImageEncoder.h

Index: include/core/SkImageEncoder.h
diff --git a/include/core/SkImageEncoder.h b/include/core/SkImageEncoder.h
index fc999dcc189d4177de70c6b0a4810bb1e0165de1..5fa69f799297560b473d922b3872c397ccfab680 100644
--- a/include/core/SkImageEncoder.h
+++ b/include/core/SkImageEncoder.h
@@ -13,6 +13,8 @@
 
 class SkBitmap;
 class SkData;
+class SkPixmap;
+class SkPixelSerializer;
 class SkWStream;
 
 class SkImageEncoder {
@@ -60,8 +62,11 @@ public:
      */
     bool encodeStream(SkWStream* stream, const SkBitmap& bm, int quality);
 
+    SkPixelSerializer* refSerializer();
+
     static SkData* EncodeData(const SkImageInfo&, const void* pixels, size_t rowBytes,
                               Type, int quality);
+    static SkData* EncodeData(const SkPixmap&, Type, int quality);
     static SkData* EncodeData(const SkBitmap&, Type, int quality);
 
     static bool EncodeFile(const char file[], const SkBitmap&, Type,
@@ -69,6 +74,12 @@ public:
     static bool EncodeStream(SkWStream*, const SkBitmap&, Type,
                            int quality);
 
+    /**
+     *  If the existing data can be re-encoded into the specified type (efficiently), this
+     *  returns that new data (which the caller must unref()). If not, this returns null.
+     */
+    static SkData* ReencodeData(SkData* encoded, Type);
+
 protected:
     /**
      * Encode bitmap 'bm' in the desired format, writing results to
@@ -78,6 +89,8 @@ protected:
      * This must be overridden by each SkImageEncoder implementation.
      */
     virtual bool onEncode(SkWStream* stream, const SkBitmap& bm, int quality) = 0;
+
+    virtual SkData* onReencodeData(SkData*) { return nullptr; }
 };
 
 // This macro declares a global (i.e., non-class owned) creation entry point