Better handle small screenshots in app banners

[Kekun]

I'm working on adding brand colors to some app, and for that I'm using the banner previewer. While doing so I noticed the app's screenshot doesn't look great in the previewer.

The app's screenshot, for reference:

That app has quite a small and narrow window, and there isn't a point in making it larger in any way as it brings nothing to the user and it just makes the app look weird: it's designed to be used as a short, squarish or narrow window.

If I make the window twice as wide we start to see some relevant details of the window, but barely and the screeshot just looks odd:

I see there's an "Is Fullscreen App" toggle in the previewer, so some apps are special-cased, maybe small/narrow apps could be special-cased too? I don't have any good idea to make the screenshot look decent and attractive in the banner.

[bertob]

For screenshots that are wide but not tall, we could probably auto-detect that from the size and switch to a layout that vertically centers it. See also CleoMenezesJr/Aurea#6

Screenshots that are tall but not wide are trickier. The only realistic option that comes to mind is showing multiple portrait screenshots next to each other. This would mean that we require apps to do some pretty specific things though:

• Have the first 2 or 3 screenshots be portrait
• Have them all be the same size
• Make sure they look good next to each other

It could work, but at that point I wonder if this stuff should be explicit in the appstream metadata rather than conventions at the Flathub level...