Made optional to switch between scalers in menu.

Centered 320x238 scaler in screen.
Improved detail of the bilinearish scaler.
Merge pull request #5 from hi-ban/master

Add an option to switch between the fast scaler and the approximately bilinear scaler.
Center the 320x238 scaler on-screen.
Improve the crispness of detail on scanlines #8 and #10 of 16-scanline blocks in the approximately bilinear scaler.