Option allow downscaling 720p/1080p to 480p

It allows 720p/1080p downscaling to 480p . Since FBO size getting smaller and thus faster .

Meanwhile , it also fixes wrong display stretching issues in some games using 720p and 1080p as well .

- Spelunky (http://www.emunewz.net/forum/showthread.php?tid=159075)
![1](https://cloud.githubusercontent.com/assets/3000282/3338768/7eec0164-f860-11e3-8622-6f52c0cd858f.jpg)

- Jetpack Joyride (http://www.emunewz.net/forum/showthread.php?tid=162886&pid=211738#pid211738)
![3](https://cloud.githubusercontent.com/assets/3000282/3338724/b46fffee-f85f-11e3-8b3e-3eaa8b6b1fe4.jpg)

- Retro city rampage (http://www.emunewz.net/forum/showthread.php?tid=162900)
![2](https://cloud.githubusercontent.com/assets/3000282/3338727/c00ce448-f85f-11e3-865c-5f6eb27ed97a.jpg)