[SRU] Demote libde265 to Suggests

[Charles]

[ Impact ]

 * We shouldn't be including an H.265 decoder in the Ubuntu squashfs.
   In Noble, there is a dependency chain in the desktop seed that pulls
   this component in:

     cups-core-drivers -> cups-daemon -> colord -> libsane1
       -> libgphoto2-6t64 -> libgd3 -> libheif -> libheif-plugin-libde265

 * By demoting libheif1's Depends on libheif-plugin-libde265 to
   Suggests, germinate will no longer pull it into the squashfs seed.

 * The practical consequence of this change is that HEIC images
   (the default photo format on iOS/macOS devices since iOS 11 and
   the default photo format for many recent Android phones) will
   not decode on a live session or freshly installed system unless the
   user explicitly installs libheif-plugin-libde265. AVIF images are
   unaffected as AV1 decoding (dav1d/aomdec) remains a hard dependency.

 * This change was already carried in ubuntu4 for the x265 encoder
   (LP: #2061089). This upload extends the same principle to the
   libde265 decoder.

[ Test Plan ]

 Verify AVIF decoding is unaffected (dav1d/aomdec still a hard dep):

      # On a system WITHOUT libheif-plugin-libde265 installed:
      $ apt-get install --no-install-recommends libheif-examples
      $ heif-convert test.avif test.png
      # Should succeed

 Verify HEIC decoding fails gracefully without the plugin:

      # On a system WITHOUT libheif-plugin-libde265 installed:
      $ heif-convert iphone-photo.heic out.png
      # Should fail with a reasonable error not a crash or silent
        corruption.

 Verify HEIC decoding still works when the plugin is installed:

      $ apt-get install libheif-plugin-libde265
      $ heif-convert iphone-photo.heic out.png
      # Should succeed and produce a correct PNG

 Verify no regression for applications using the gdk-pixbuf loader:

      # With libheif-plugin-libde265 installed:
      $ gdk-pixbuf-query-loaders | grep heif
      $ eog iphone-photo.heic # or any GTK image viewer
      # Should display correctly

 Once new ISOs are available, ensure the problematic packages have not been pulled into the squashfs,

      sudo mount -o loop,ro ubuntu-release.iso mnt
      sudo mount -o loop,ro mnt/casper/minimal.standard.squashfs squashfs
      # Ensure libheif-plugin-x265 and libheif-plugin-libde265 are not seeded
      dpkg-query --admindir=./squashfs/var/lib/dpkg -W 'libheif*'

[ Where problems could occur ]

 * Applications that rely on libheif for HEIC thumbnailing or display
   (e.g. Nautilus via heif-thumbnailer or heif-gdk-pixbuf, GNOME
   Photos, Shotwell) will silently fail to open or thumbnail HEIC
   images on fresh installs. Users coming from iPhones are likely to
   notice this. The failure mode should be graceful (error returned to
   caller), not a crash, but this is worth confirming.

 * If any package in main or the desktop seed has an undocumented
   indirect requirement on HEIC decoding being available, that
   functionality will silently regress. The dependency chain from
   libgphoto2 makes this plausible. software that transfers photos
   from iOS cameras via gphoto2 and then previews them could be
   affected.

[ Other Info ]
 * A similar change was already carried in ubuntu4 for the x265
   encoder (LP: #2061089). This upload extends the same principle to
   the libde265 decoder.

 * Questing also seeds the decoder by default. A similar change has
   already been released for Resolute
   (https://launchpad.net/ubuntu/+source/libheif/1.21.2-3)

 * The libde265 plugin itself is still built and still shipped in the
   archive in the libheif-plugin-libde265 binary package; only its
   installation becomes optional.

 * Users who need HEIC support can restore it with:
     sudo apt-get install libheif-plugin-libde265

[Jean-Baptiste Lallement (jibel)]

It landed in resolute, closing the corresponding task.