Allow block transfer (download only)

It is a faster alternative as read to framebuffer memory .It helps those games which require block transfer download.

For example

- Ys Seven Mini map (Read to framebuffer memory mode , 113 FPS)
![ulus10551_00001](https://cloud.githubusercontent.com/assets/3000282/2869876/7e7c6b5c-d299-11e3-8707-3008bfc8290e.jpg)

- Ys Seven Mini map (Buffered rendering mode with block transfer enabled , 240 FPS)
![ulus10551_00000](https://cloud.githubusercontent.com/assets/3000282/2869877/847e7d7e-d299-11e3-8668-b1ca2d5ab61f.jpg)