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

src/core/SkNextID.h

Index: src/core/SkNextID.h
diff --git a/src/core/SkNextID.h b/src/core/SkNextID.h
new file mode 100644
index 0000000000000000000000000000000000000000..570fd946dedf5b665091a86dc1c7aad24b76f97f
--- /dev/null
+++ b/src/core/SkNextID.h
@@ -0,0 +1,21 @@
+/*
+ * Copyright 2015 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef SkNextID_DEFINED
+#define SkNextID_DEFINED
+
+#include "SkTypes.h"
+
+class SkNextID {
+public:
+    /**
+     *  Shared between SkPixelRef's generationID and SkImage's uniqueID
+     */
+    static uint32_t ImageID();
+};
+
+#endif