add encodeData() to SkImageEncoder, and add encoding to SkImage

src/image/SkImage_Base.h

Index: src/image/SkImage_Base.h
diff --git a/src/image/SkImage_Base.h b/src/image/SkImage_Base.h
index d8e64a9a7516dc7b53b4e6dd80b603c1d679c990..0b3ddb43b74b7efaeaad685379f26ee446c42523 100644
--- a/src/image/SkImage_Base.h
+++ b/src/image/SkImage_Base.h
@@ -16,6 +16,10 @@ public:
 
     virtual void onDraw(SkCanvas*, SkScalar x, SkScalar y, const SkPaint*) = 0;
     virtual GrTexture* onGetTexture() { return NULL; }
+    
+    // return a read-only copy of the pixels. We promise to not modify them,
+    // but only inspect them (or encode them).
+    virtual bool getROPixels(SkBitmap*) const { return false; }
 
 private:
     typedef SkImage INHERITED;