mark all SkImage methods const, so we can make it thread-safe

include/core/SkImage.h

Index: include/core/SkImage.h
diff --git a/include/core/SkImage.h b/include/core/SkImage.h
index 04e184370f73400b29c3201e24de0a101801bed0..581129e7207fd58a3311f9d161a372a6eefdd46a 100644
--- a/include/core/SkImage.h
+++ b/include/core/SkImage.h
@@ -62,7 +62,7 @@ public:
                                 SkShader::TileMode,
                                 const SkMatrix* localMatrix = NULL) const;
 
-    void draw(SkCanvas*, SkScalar x, SkScalar y, const SkPaint*);
+    void draw(SkCanvas*, SkScalar x, SkScalar y, const SkPaint*) const;
 
     /**
      *  Draw the image, cropped to the src rect, to the dst rect of a canvas.
@@ -71,7 +71,7 @@ public:
      *
      *  See SkCanvas::drawBitmapRectToRect for similar behavior.
      */
-    void draw(SkCanvas*, const SkRect* src, const SkRect& dst, const SkPaint*);
+    void draw(SkCanvas*, const SkRect* src, const SkRect& dst, const SkPaint*) const;
 
     /**
      *  If the image has direct access to its pixels (i.e. they are in local