When a surface is backed by an external render target force a copy on image snap

src/gpu/GrGpuResourcePriv.h

Index: src/gpu/GrGpuResourcePriv.h
diff --git a/src/gpu/GrGpuResourcePriv.h b/src/gpu/GrGpuResourcePriv.h
index 5324dccc22ad2842d76dbda15df381cadc54022e..1884e73961826945c022e568156662b4a865d9b2 100644
--- a/src/gpu/GrGpuResourcePriv.h
+++ b/src/gpu/GrGpuResourcePriv.h
@@ -50,6 +50,11 @@ public:
         return ret;
     }
 
+    /**
+     * Is the resource object wrapping an externally allocated GPU resource?
+     */
+    bool isExternal() const { return fResource->isExternal(); }
+
     /** 
      * If this resource can be used as a scratch resource this returns a valid scratch key.
      * Otherwise it returns a key for which isNullScratch is true. The resource may currently be