Add text shadow to Yes/No/Enter/Back button


Real PSP
![msg_psp](https://f.cloud.github.com/assets/3000282/696022/fc1c2a32-dcdd-11e2-9011-916721afffb8.png)

PPSSPP
![screen00010](https://f.cloud.github.com/assets/3000282/696023/063cf6ae-dcde-11e2-9bb0-04d44c75232b.jpg)