add ability to get FBO ID to Surface

src/gpu/gl/GrGLRenderTarget.h

Index: src/gpu/gl/GrGLRenderTarget.h
diff --git a/src/gpu/gl/GrGLRenderTarget.h b/src/gpu/gl/GrGLRenderTarget.h
index 2113a800afd677c2f401ff90b92637858d423641..1e6dc7f767edd9a738415f7b9b2d69da50e67263 100644
--- a/src/gpu/gl/GrGLRenderTarget.h
+++ b/src/gpu/gl/GrGLRenderTarget.h
@@ -55,6 +55,8 @@ public:
         }
     }
 
+    GrBackendObject getRenderTargetHandle() const override { return fRTFBOID; }
+
     /** When we don't own the FBO ID we don't attempt to modify its attachments. */
     bool canAttemptStencilAttachment() const override {
         return kCached_LifeCycle == fRTLifecycle || kUncached_LifeCycle == fRTLifecycle;