reftest_compare_surfaces: Report how much the images differ

In unattended/non-interactive/autobuilder environments where the images
are not trivially accessible, this provides a way to distinguish between
totally different rendering and more subtle issues.

Signed-off-by: Simon McVittie <smcv@debian.org>
Forwarded: https://gitlab.gnome.org/GNOME/gtk/-/merge_requests/3195
Applied-upstream: no, upstream want reftests to be a strict pass/fail with identical results required

Gbp-Pq: Name reftest_compare_surfaces-Report-how-much-the-images-diffe.patch

gtk4 (4.6.6+ds-1) unstable; urgency=medium

  [ Jeremy Bicha ]
  * New upstream release (LP: #1980742)

  [ Simon McVittie ]
  * d/tests/installed-tests: Fail on unset variable references
  * d/tests/installed-tests: Fail if no tests are found

  [ Evangelos Ribeiro Tzaras ]
  * Bump required glib build dependency

[dgit import unpatched gtk4 4.6.6+ds-1]

Import gtk4_4.6.6+ds.orig.tar.xz

[dgit import orig gtk4_4.6.6+ds.orig.tar.xz]

Import gtk4_4.6.6+ds-1.debian.tar.xz

[dgit import tarball gtk4 4.6.6+ds-1 gtk4_4.6.6+ds-1.debian.tar.xz]