两个PNGButton类，一个继承与CWnd，一个继承与CButton。实现按钮透明，即按钮上的图片是可有透明度的，方便不规则图形的显示，并且可给鼠标悬停、点击等操作赋予不同的图片(There are two PNGButton classes, one is inheritance and CWnd, one is inheritance and the other is CButton. The realization button is transparent, that is, the picture on the button is transparent, easy to display the irregular graphics, and can give different pictures to the operation of hover, click and so on)