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 62dc85059a1668f20050d155168e4af18711cd1a..5da1c948783ffdae44c984885e6ceaf7e9e02f32 100644
--- a/src/gpu/GrGpuResourcePriv.h
+++ b/src/gpu/GrGpuResourcePriv.h
@@ -50,7 +50,12 @@ public:
         return SkBudgeted(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
      * used as a uniquely keyed resource rather than scratch. Check isScratch().