The bug is in fact in GTK. "image-missing" is supposed to never missing since
GTK embeds an icon as GResource. But it turns out that the code is buggy. So if
an icon is missing from GIMP theme, then system theme, then "icon-missing" icon
as well, GIMP crashes.
See: https://gitlab.gnome.org/GNOME/gtk/-/issues/3526
As a workaround for now, let's at least add our own "image-missing", at least in
our main themes (Color and Symbolic, not Legacy). This will make GIMP not
crashing at least by default when another icon is missing.
Of course a crash could still happen if one use Legacy or a third-party theme
and if their system theme doesn't contain "image-missing" as well, until GTK is
fixed.
196f1d6e95
