Bug#1079547: gtk4: color mix test fails on multiple architectures

[Jeremy Bícha]

Source: gtk4
Version: 4.15.5+ds-2
Severity: important
Forwarded: https://gitlab.gnome.org/GNOME/gtk/-/issues/6907

A new test added in gtk4 4.15.* is failing on multiple architectures.
I skipped the test on Debian and reported the issue upstream. Perhaps
it's a rounding or precision issue. It was unclear to me whether the
test assumptions are broken or whether the css parsing code needs to
be adjusted.

Thank you,
Jeremy Bícha