Fix Flathub checks

[divVerent]

https://flathub.org/apps/io.github.divverent.aaaaxy

when logged in, shows 3 failing checks:

• Has primary brand colors
  • Next release will fix that, already taken care of by adding the missing tag
• Doesn't fill too much or too little of the canvas (https://docs.flathub.org/docs/for-app-authors/metainfo-guidelines/quality-guidelines/#reasonable-footprint)
  • Still to be done
  • While at it, may also want to install the SVG file as an icon instead
  • Gotta check if anything uses the SVG right now and will need adjustments too (e.g. macOS or iOS)
• Reasonable window size
  • Next release will fix that, already taken care of by downscaling the images

[divVerent]

Seems like we need the icon to be downscaled to 13/16 its current size. It then just fits the rectangle.

[divVerent]

Did this scaling factor in the SVG file now. Will soon send another release to flathub to try this out.

[divVerent]

Remaining checks that fail:

• Icon: good contrast on dark backgrounds
  • I thought I did that but there are some things that may still be possible. Keeping open.
• Icon: In line with contemporary styles
  • This is very intentional, so, this is a wontfix.
• Screenshots: reasonable window size
  • Again I thought this has been addressed, so I gotta figure out what that check really does; they ask for "1000x700 pixels or smaller" and they are 960x540 now, so no idea.