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