Don't enter seatop_move_floating when fullscreen

Currently, a floating window that's been fullscreened can send us
xdg_toplevel::move, and we'll enter seatop_move_floating, which lets us
drag the surface around while it's fullscreen. We don't want
this--fullscreen surfaces should always be aligned to the screen--so add
the same check that seatop_default already does when entering this mode.

Tested with Weston's weston-fullscreen demo, which sends a move request
if you click anywhere on its surface.

sway/desktop/xdg_shell.c

@@ -360,7 +360,8 @@ static void handle_request_move(struct wl_listener *listener, void *data) {
 	struct sway_xdg_shell_view *xdg_shell_view =
 		wl_container_of(listener, xdg_shell_view, request_move);
 	struct sway_view *view = &xdg_shell_view->view;
-	if (!container_is_floating(view->container)) {
+	if (!container_is_floating(view->container) ||
+			view->container->pending.fullscreen_mode) {
 		return;
 	}
 	struct wlr_xdg_toplevel_move_event *e = data;

sway/desktop/xwayland.c

@@ -594,7 +594,8 @@ static void handle_request_move(struct wl_listener *listener, void *data) {
 	if (!xsurface->mapped) {
 		return;
 	}
-	if (!container_is_floating(view->container)) {
+	if (!container_is_floating(view->container) ||
+			view->container->pending.fullscreen_mode) {
 		return;
 	}
 	struct sway_seat *seat = input_manager_current_seat();