unify pixelref and image ID space, so we can share IDs when we share pixels

include/core/SkImage.h

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkImage_DEFINED
 #define SkImage_DEFINED
 
@@ skipping 265 matching lines @@
      *  a raster-backed bitmap (texture-backed bitmaps are DEPRECATED, and not supported here).
      *
      *  If the mode is kRO (read-only), the resulting bitmap will be marked as immutable.
      *
      *  On succcess, returns true. On failure, returns false and the bitmap parameter will be reset
      *  to empty.
      */
     bool asLegacyBitmap(SkBitmap*, LegacyBitmapMode) const;
     
 protected:
-    SkImage(int width, int height) :
-        fWidth(width),
-        fHeight(height),
-        fUniqueID(NextUniqueID()) {
-
-        SkASSERT(width > 0);
-        SkASSERT(height > 0);
-    }
+    SkImage(int width, int height, uint32_t uniqueID);
 
 private:
     const int       fWidth;
     const int       fHeight;
     const uint32_t  fUniqueID;
 
-    static uint32_t NextUniqueID();
-
     typedef SkRefCnt INHERITED;
 };
 
 #endif