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

BUG=
R=scroggo@google.com

Review URL: https://codereview.chromium.org/15002004

git-svn-id: http://skia.googlecode.com/svn/trunk@9193 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/src/image/SkImage_Gpu.cpp b/src/image/SkImage_Gpu.cpp
index 451ad07..b6e4211 100644
--- a/src/image/SkImage_Gpu.cpp
+++ b/src/image/SkImage_Gpu.cpp
@@ -22,6 +22,10 @@
 
     virtual void onDraw(SkCanvas*, SkScalar x, SkScalar y, const SkPaint*) SK_OVERRIDE;
     virtual GrTexture* onGetTexture() SK_OVERRIDE;
+    virtual bool getROPixels(SkBitmap*) const SK_OVERRIDE {
+        // TODO
+        return false;
+    }
 
     GrTexture* getTexture() { return fTexture; }