add ability to get FBO ID to Surface

include/gpu/GrRenderTarget.h

Index: include/gpu/GrRenderTarget.h
diff --git a/include/gpu/GrRenderTarget.h b/include/gpu/GrRenderTarget.h
index 67dcd4f4c256b395e35e7b195130f7dd30cc5e7e..8983d53a251847325254441c86492436d555c088 100644
--- a/include/gpu/GrRenderTarget.h
+++ b/include/gpu/GrRenderTarget.h
@@ -138,6 +138,12 @@ public:
     };
     virtual ResolveType getResolveType() const = 0;
 
+    /**
+     *  Return the native ID or handle to the rendertarget, depending on the
+     *  platform. e.g. on OpenGL, return the FBO ID.
+     */
+    virtual GrBackendObject getRenderTargetHandle() const = 0;
+
     // Provides access to functions that aren't part of the public API.
     GrRenderTargetPriv renderTargetPriv();
     const GrRenderTargetPriv renderTargetPriv() const;