Add support for 4:3 resolution

[siffland]

I have one of the ipad retina display's the same use used in Laser Bear's "crt" kit and on aliepxress.

Laser Bear
aliexpress

The issue is the resolution is 2048x1536 (4:3). This results in the image being squished. Is there a way to support this resolution, or even a variable and i can compile a version for myself. In Mister, they created video_mode=13 for this.

The resolution the nestang displays on the display is 1280x720. Maybe a 1024x768 (not sure if the tang 20k can do 1600x1200).

Mister video modes

It would be a sweet setup with the nestang (and snestang) and this display.

[nand2mario]

We can address this once we have a way to save options (#47)