This update for gthumb fixes the following issues:

Update to version 3.12.2 (boo#1209655 boo#1209656):

- Fixed possible crash when changing directory during
  thumbnails creation.
- Added support for larger thumbnail sizes: 512, 768 and
  1024 pixels.

Update to version 3.12.1:

- PNG loader ignores gAMA.
- Transparent webp images are rendered incorrectly.
- Crash when dropping files on an empty folder/catalog.
- Fixed location loading after mounting volume.
- JPEG image dimensions not displayed for files inside the
  trash.
- Infinite Loop when viewing more than 50 thumbnails.
- X11 window title is not set.
- Copy release notes on build from NEWS to metainfo file.
- Fixed build if appstream >= 0.14.6 is not available.
- Fix incorrect usage of shared_module in extensions.
- cli: mention that user can open files and/or directories.
- Updated translations.