xdg-shell: fetch existing extents when surface is created

The surface might alredy have non-0 extents due to the use of
wp_viewporter.

src/ifs/wl_surface/xdg_surface/xdg_toplevel.rs

@@ -157,6 +157,7 @@ impl XdgToplevel {
         toplevel_data
             .content_type
             .set(surface.surface.content_type.get());
+        toplevel_data.pos.set(surface.extents.get());
         Self {
             id,
             state: state.clone(),