Fix some cases where the cursor doesn't update

cursor_set_image only uploads the named image if it doesn't match the
previous named image. This means when setting the cursor image to a
surface as given by a client, we have to clear the currently stored
image.

sway/input/cursor.c

@@ -726,6 +726,7 @@ static void handle_request_set_cursor(struct wl_listener *listener,
 
 	wlr_cursor_set_surface(cursor->cursor, event->surface, event->hotspot_x,
 		event->hotspot_y);
+	cursor->image = NULL;
 	cursor->image_client = focused_client;
 }