Fix displaying of embedded `Window`

Fix Rect of texture to take window and stretch transform into account.

There is no need for `viewport_attach_to_screen` for embedded windows,
since their display is handled via `Viewport::_sub_window_update`.

scene/main/viewport.cpp

@@ -348,7 +348,13 @@ void Viewport::_sub_window_update(Window *p_window) {
 		close_icon->draw(sw.canvas_item, r.position + Vector2(r.size.width - close_h_ofs, -close_v_ofs));
 	}
 
-	RS::get_singleton()->canvas_item_add_texture_rect(sw.canvas_item, r, sw.window->get_texture()->get_rid());
+	const Transform2D xform = sw.window->window_transform * sw.window->stretch_transform;
+	Rect2 vr = xform.xform(sw.window->get_visible_rect());
+	vr.position += p_window->get_position();
+	if (vr != r) {
+		RS::get_singleton()->canvas_item_add_rect(sw.canvas_item, r, Color());
+	}
+	RS::get_singleton()->canvas_item_add_texture_rect(sw.canvas_item, vr, sw.window->get_texture()->get_rid());
 }
 
 void Viewport::_sub_window_grab_focus(Window *p_window) {

scene/main/window.cpp

@@ -1239,7 +1239,7 @@ void Window::_update_viewport_size() {
 
 	if (window_id != DisplayServer::INVALID_WINDOW_ID) {
 		RenderingServer::get_singleton()->viewport_attach_to_screen(get_viewport_rid(), attach_to_screen_rect, window_id);
-	} else {
+	} else if (!is_embedded()) {
 		RenderingServer::get_singleton()->viewport_attach_to_screen(get_viewport_rid(), Rect2i(), DisplayServer::INVALID_WINDOW_ID);
 	}