Only connect to BufferInterface::aboutToBeDestroyed once

It is allowed to call surface_attach multiple times with the same
buffer. We want to only connect once.

Found whilst debugging a freeze, going through the list of
BufferInterface connections I had multiple to the same SurfaceInterface.

CCBUG: 428361

src/wayland/surface_interface.cpp

@@ -325,8 +325,8 @@ void SurfaceInterfacePrivate::surface_attach(Resource *resource, struct ::wl_res
                 current.buffer->unref();
                 current.buffer = nullptr;
             }
-        }
-    );
+        },
+        Qt::UniqueConnection);
 }
 
 void SurfaceInterfacePrivate::surface_damage(Resource *, int32_t x, int32_t y, int32_t width, int32_t height)