Support new Snap banner format
The existing Snap banners are set by looking for specially named screenshots to use as the background and icon images. These should match the GNOME Software banner format and have the name/summary text overlaid on them.
Snap now has a native banner format which is a 3:1 ratio and is not designed to have text overlaid on it. The whole image is intended to be seen (i.e. can't be clipped). You can generate CSS to show these by setting the background-size to 'contain' and settings the text color to 'transparent'. It doesn't fit the space well:
Some ideas to supporting this:
- Allowing banners to indicate they're in a different format, and request additional height and overlaid text to be removed.
- Allowing banners to indicate their preferred dimensions and showing multiple banners side-by-side if space is available.
- Not supporting snap banners.