[Libretro] Add 4:3 aspect ratio to Game Gear

[dexwerx]

I think this could be 4:3 given the non-square pixels of the GG LCD.

[drhelius]

You're right, I'll take a look, maybe with an option to choose the one you prefer.

[dexwerx]

Certainly not urgent - RA allows you to set the aspect ratio as an override. The core as is looks and and runs great in 4:3.