Fix dormant cursor when using multiple seats

The cursor's image would be removed or set when the seat's capabilities
were updated, but there was nothing to prevent the image from being set
at other times.

sway/input/cursor.c

@@ -1220,6 +1220,9 @@ static void handle_request_set_cursor(struct wl_listener *listener,
 
 void cursor_set_image(struct sway_cursor *cursor, const char *image,
 		struct wl_client *client) {
+	if (!(cursor->seat->wlr_seat->capabilities & WL_SEAT_CAPABILITY_POINTER)) {
+		return;
+	}
 	if (!image) {
 		wlr_cursor_set_image(cursor->cursor, NULL, 0, 0, 0, 0, 0, 0);
 	} else if (!cursor->image || strcmp(cursor->image, image) != 0) {