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

include/core/SkImage.h

Index: include/core/SkImage.h
diff --git a/include/core/SkImage.h b/include/core/SkImage.h
index 2b759b800387882577b6cad8a7c029b44b56a373..e89ffa5e59a9037a76e8739fbed6df71413e05a6 100644
--- a/include/core/SkImage.h
+++ b/include/core/SkImage.h
@@ -283,22 +283,13 @@ public:
     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;
 };