New composite based captcha image

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recaptcha的验证码新增了alpha composite的新机制取代干扰线，今天用了一些时间在YAN上也实现了这种绘图机制。

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使用Java2D的AlphaComposite实现，选用的Rule为alpha 1.0的SrcOut，即通过公式

Ar = As * (1 – Ad )
Cr = Cs * (1 – Ad )
用语言描述就是叠加区域的透明度为0. 使用这种机制必须采用BufferedImage.TYPE_INT_ARGB的图像，并且输出支持alpha通道的格式。

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