Reimplement window closing anim in an efficient way

- Keep a root surface cache to be accessible in surface destroyed()
- Only snapshot during / right before closing, rather than every frame
- Store textures rather than elements to handle scale and alpha properly

src/handlers/xdg_shell.rs

@@ -382,6 +382,9 @@ impl XdgShellHandler for State {
         let window = mapped.window.clone();
         let output = output.clone();
 
+        self.backend.with_primary_renderer(|renderer| {
+            mapped.render_and_store_snapshot(renderer);
+        });
         self.backend.with_primary_renderer(|renderer| {
             self.niri
                 .layout