swaynag: fix pointer management

Currently on master, swaynag will retrieve a pointer instance whenever
the capabilities change and WL_SEAT_CAPBILITY_POINTER is set. The
pointer instances were never being destroyed so swaynag received events
multiple times due to having several instances of the pointer.

This fixes it so if there is already a pointer instance, swaynag does
not attempt to retrieve another. Additionally, if the pointer
capability is removed, the pointer instance is destroyed.

swaynag/swaynag.c

@@ -239,10 +239,14 @@ static struct wl_pointer_listener pointer_listener = {
 static void seat_handle_capabilities(void *data, struct wl_seat *wl_seat,
 		enum wl_seat_capability caps) {
 	struct swaynag *swaynag = data;
-	if ((caps & WL_SEAT_CAPABILITY_POINTER)) {
+	bool cap_pointer = caps & WL_SEAT_CAPABILITY_POINTER;
+	if (cap_pointer && !swaynag->pointer.pointer) {
 		swaynag->pointer.pointer = wl_seat_get_pointer(wl_seat);
 		wl_pointer_add_listener(swaynag->pointer.pointer, &pointer_listener,
 				swaynag);
+	} else if (!cap_pointer && swaynag->pointer.pointer) {
+		wl_pointer_destroy(swaynag->pointer.pointer);
+		swaynag->pointer.pointer = NULL;
 	}
 }