Fall back to use region instead of viewport to estimate drawing size

Although the region here is not truely correct however it definitely much better than using viewport as it broken number of games in terms of graphic.

This fall back fixes multiple games with graphic issues previously in buffered rendering mode

1. MotoGP (Incorrect viewport)  Issue #702 
![screen00044](https://f.cloud.github.com/assets/3000282/756818/c3f68254-e649-11e2-8ca7-ad3706341822.jpg)

2. FF Type-0 (Dynamic Shadowing issue)  Issue #1272 
![screen00045](https://f.cloud.github.com/assets/3000282/756826/93396aea-e64a-11e2-9e03-3252c48491bd.jpg)

3. Castlevania The Dracula X Chronicles (Incorrect background size)
![screen00046](https://f.cloud.github.com/assets/3000282/756827/a2a3b3dc-e64a-11e2-88f4-117aa28cf7ac.jpg)


Meanwhile , the workaround of multiple with 2.0 in HW/SW mode can be reverted as the fall back here fixes issue in Castlevania as well.