change offset to xy for pixelref subsetting

include/core/SkPixelRef.h

Index: include/core/SkPixelRef.h
diff --git a/include/core/SkPixelRef.h b/include/core/SkPixelRef.h
index 48114357211598fcab0d146d849a21f1dd72ef65..e0bf8866c69b620631cd65a36bf9ed148ed0edbe 100644
--- a/include/core/SkPixelRef.h
+++ b/include/core/SkPixelRef.h
@@ -69,6 +69,8 @@ public:
     */
     SkColorTable* colorTable() const { return fRec.fColorTable; }
 
+    size_t rowBytes() const { return fRec.fRowBytes; }
+
     /**
      *  To access the actual pixels of a pixelref, it must be "locked".
      *  Calling lockPixels returns a LockRec struct (on success).