backend/wayland: fix touch device not added on startup

We were firing the new_input signal on backend initialization,
before the compositor had the chance to add a listener for it.

Mimick what's done for wl_keyboard: if the backend hasn't been
started, delay wl_touch initialization.

Closes: https://gitlab.freedesktop.org/wlroots/wlroots/-/issues/3473

backend/wayland/backend.c

@@ -424,6 +424,10 @@ static bool backend_start(struct wlr_backend *backend) {
 			init_seat_keyboard(seat);
 		}
 
+		if (seat->wl_touch) {
+			init_seat_touch(seat);
+		}
+
 		if (wl->tablet_manager) {
 			init_seat_tablet(seat);
 		}

backend/wayland/seat.c

@@ -212,7 +212,7 @@ static const struct wlr_touch_impl touch_impl = {
 	.name = "wl-touch",
 };
 
-static void init_seat_touch(struct wlr_wl_seat *seat) {
+void init_seat_touch(struct wlr_wl_seat *seat) {
 	assert(seat->wl_touch);
 
 	char name[128] = {0};
@@ -333,7 +333,9 @@ static void seat_handle_capabilities(void *data, struct wl_seat *wl_seat,
 		wlr_log(WLR_DEBUG, "seat '%s' offering touch", seat->name);
 
 		seat->wl_touch = wl_seat_get_touch(wl_seat);
-		init_seat_touch(seat);
+		if (backend->started) {
+			init_seat_touch(seat);
+		}
 	}
 	if (!(caps & WL_SEAT_CAPABILITY_TOUCH) && seat->wl_touch != NULL) {
 		wlr_log(WLR_DEBUG, "seat '%s' dropping touch", seat->name);

include/backend/wayland.h

@@ -144,6 +144,8 @@ void init_seat_pointer(struct wlr_wl_seat *seat);
 void finish_seat_pointer(struct wlr_wl_seat *seat);
 void create_pointer(struct wlr_wl_seat *seat, struct wlr_wl_output *output);
 
+void init_seat_touch(struct wlr_wl_seat *seat);
+
 void init_seat_tablet(struct wlr_wl_seat *seat);
 void finish_seat_tablet(struct wlr_wl_seat *seat);