gimp: Update to 2.10.28

Changelog:
Overview of Changes from GIMP 2.10.26 to GIMP 2.10.28
=====================================================

Translations:

  - 10 translations were updated: Catalan, Chinese (China), Finnish,
    Italian, Polish, Russian, Slovenian, Spanish, Swedish and Ukrainian.

Build:

  - Fix uninstalled white-border prelight file.

Overview of Changes from GIMP 2.10.24 to GIMP 2.10.26
=====================================================

Core:

  - Dashboard now has memory support in OpenBSD.
  - Default shortcuts Shift+[ and ] for tool size changed to { and }.
  - Performance improvements for GIMP on macOS Big Sur: these
    improvements were actually already applied in our macOS packages
    since GIMP 2.10.22, but were applied upstream directly only now.

User Interface:

  - Dark theme: improve accessibility of GtkRadioButton by adding a
    white border on mouse-hover.
  - Gray theme: set light background for selected text in Layers and
    Paths dockable dialogs to make text visible.
  - Dark, Gray and Light themes: remove 3D shadow box around eye and
    link toggles in Layers, Channels, and Paths dockable dialog tree
    views.

Plug-ins:

  - Improved DDS support and fix some red/blue bit swap for RGB10A2 DDS.
    The plug-in version is incremented so that it is able to catch and
    correct previously incorrect RGB10A2 images exported by older
    versions of our plug-in.
  - DDS files with "L16" - a 16 bit luminance channel - are now loaded
    in 16-bit.
  - DICOM images:
    * Support for planar configuration.
    * Support for deprecated big endian transfer.
  - TIFF images:
    * Thumbnail storing now done by storing the thumbnail as the second
      page in the file (through Exiv2) and setting metadata
      "Exif.Thumbnail.NewSubfileType" to 1 (reduced resolution image)
      instead of storing the thumbnail as a subifd. This was done
      because of a Windows bug locking TIFF files with thumbnail stored
      as subfid.
    * Symmetrically, loading will ignore pages marked as "reduced
      resolution image" (i.e. it will consider them as thumbnails), as
      well as try to guess if a page (without subfile type) is a
      thumbnail when it meets following criteria: second page with YCbCr
      PhotometricInterpretation, old style jpeg compression while the
      first page has a different PhotometricInterpretation or
      compression.

Script-fu:

  - New (dir-make "/dir/name" mode) function to create a directory.

Translations:

  - New translations for the Windows installer: Vietnamese, Lithuanian.
  - 13 translations were updated: Catalan, Chinese (China), Croatian,
    Dutch, German, Lithuanian, Polish, Russian, Slovenian, Spanish,
    Swedish, Ukrainian and Vietnamese.

Build:

  - On Windows, *.rs file extension is not associated with SUN Raster
    images anymore. The reason is that this file extension is mostly
    used for Rust code files nowadays. If Windows could detect file
    formats with "magic numbers" (i.e. byte identifiers), it would not
    be a problem, but since it relies apparently only the extension,
    it's better to remove this association.
  - Oppositely on Linux and other Unix-like systems using desktop files,
    add the image/x-sun-raster MimeType which was forgotten (there,
    detection should be fine and not depend on file extension).
  - gimp30-tips.mo is not installed anymore (only used during the build
    to generate gimp-tips.xml with multiple language support).
  - --enable-check-update now has an "auto" value, which is the new
    default. It is equivalent to "yes" for Windows and macOS and "no" in
    all other cases.
  - New unit test to check that localizations listed in the installer
    script match available po files in po-windows-installer/.
  - "msys*" host value now detected as Windows builds.
  - Bumping minimum GEGL to version 0.4.32.


(ryoon)