icontheme: allow directories with mtime 0
icontheme: allow directories with mtime 0

In order to provide a constant mtime between OS build and deploy time,
while also maintaining a hardlink content-addressed model independent of
timestamps, ostree sets all mtimes to 0.

The icon cache code currently ignores directories with mtime 0, assuming
they don't exist.

Track directory existence in a more precise way.

https://bugzilla.gnome.org/show_bug.cgi?id=745052

Distrotech/gtk,Distrotech/gtk,Distrotech/gtk,Distrotech/gtk,Distrotech/gtk