trivial: Allow local files to contribute screenshots
Always resize the screenshots to the required UI size

This will look bad for non-standard screenshots, but non-padded images will
only be loaded when using local files.

Also, we'll be using the huge 'source' screenshots for the thumbnails, but
that's probably okay.
Add --prefer-local to prefer the local data to the distro AppStream metadata

Users can also use GNOME_SOFTWARE_PREFER_LOCAL=1 in the environment settings.

Resolves: https://bugzilla.gnome.org/show_bug.cgi?id=732466