Zeroize destination alpha as well when stencil test disabled.

This fixes the artifact on FF Type-0 and 3rd birthday while Persona still good at rendering the title screen correctly with alpha blending here.

![screen00067](https://f.cloud.github.com/assets/3000282/1144707/9f1ae9ce-1dc2-11e3-99d6-56c6ad8638b2.jpg)
![screen00066](https://f.cloud.github.com/assets/3000282/1144708/a28f5158-1dc2-11e3-8a7c-3da64ae13f5c.jpg)
![screen00065](https://f.cloud.github.com/assets/3000282/1144709/a837255e-1dc2-11e3-8e4c-8e6b5b9cb322.jpg)