container: predict window size for initial configure

previously, windows were allowed determine their own initial dimensions,
then fed a resize afterwards. we now look up the parent container and
try to predict + report the real size up front to minimise visual
artifacting on spawn/destroy

src/ifs/wl_surface/xdg_surface/xdg_toplevel.rs

@@ -780,7 +780,8 @@ impl XdgSurfaceExt for XdgToplevel {
     fn initial_configure(self: Rc<Self>) {
         let rect = self.xdg.absolute_desired_extents.get();
         if rect.is_empty() {
-            self.send_configure(0, 0);
+            let (w, h) = self.state.predict_tiled_body_size().unwrap_or((0, 0));
+            self.send_configure(w, h);
         } else {
             self.send_configure_checked(rect.width(), rect.height());
         }