说明：  使得最近打开的MATLAB图形开始下雪，如果没有已经打开的窗口，则首先创建一个窗口。
(LETITSNOW- Make the figure snow! Usage: ------ letitsnow - Create snow in the CURRENT figure. If no figure exists, an empty figure will be created. letitsnow(figure_hdl) - Create snow in the figure represented by figure_hdl letitsnow(figure_hdl, color)- Create snow with the specified color color is a scalar ranged 0~1)