fix: panics on output removal

Despawning the output when its global was removed proved to be
overzealous, as the Dispatch of `zxdg_output_v1` still needed it for
cleanup. Instead, only the `OutputScaleFactor` element is removed, and
every surface entity with an `OnOutput` referencing the removed output,
so scaling events sent to surfaces on non-existent outputs did not panic
or use the provided scale as if an output still existed for it.

src/server/event.rs

@@ -983,7 +983,7 @@ impl Event for client::wl_touch::Event {
     }
 }
 
-#[derive(Copy, Clone)]
+#[derive(Copy, Clone, PartialEq, Eq)]
 pub(super) struct OnOutput(pub Entity);
 struct OutputName(String);
 fn get_output_name(output: Option<&OnOutput>, world: &World) -> Option<String> {