inclusion of 3:2 screen ratio for side-by-side hybrid layout

[LigerHero]

Currently only two screen ratios are provided on the standard melon DS core and this updated version, they are 2:1 and 3:1. The resolution of the DS is 256x192 on both top and bottom screens, which fit perfectly side by side on a on a 720p screen if you increase the resolution of one DS screen to 3x and the second DS screen to 2x. Here's a reddit thread of someone else illustrating this layout with a picture. https://www.reddit.com/r/EmuDeck/comments/14ksz3m/it_should_be_possible_to_run_ds_games/

The DS as well as the Nintendo handhelds that released before it all had square pixel grid LCD displays, and their games were designed with that in mind, so reproducing them at higher resolutions with integer scale is incredibly important to fully appreciate the sprite work of 2D games.

[JesseTG]

I have some ideas for generalizing the screen layout system; when I do, I'll be sure to make this one of the included ratios.