backend-vnc: make sure to finish frames with timestamps in the past

Round up the ms delay to make sure that the finish_frame_timer always expires after the next frame_time. That way, finish_frame_handler() never passes a timestamp in the future to weston_output_finish_frame().

Setting frame_time into the future risks hitting an assert in weston_output_finish_frame(), when it is called from start_repaint_loop within the frame interval.