Small screenshots in metadata are upscaled

[A6GibKm]

If an screenshot is too small, it will be upscaled to fit in a minimum size at flathub. This can be problematic for apps that have a fixed size.

See https://gitlab.gnome.org/World/design/emblem/-/issues/23 for more information.

cc @razzeee

[razzeee]

Current layout assumes, that you can fill 468 pixels of height