Force aspect ratio on screen resize

[jjant]

Description

Currently you can resize the window and "break" the 1:1 aspect ratio, this distorts the pixels.
I think we want to allow resizing the screen arbitrarily but fit to the smallest dimension: in a 600px x 800px window, we'd show a (vertically) centered 600px x 600px image.

[lesleyrs]

I would also be ok with not allowing resizing in the meantime. Or an option for it.