icon-theme: Better handling of unscaled icon directories

icon-theme: Better handling of unscaled icon directories

If an icon is in a Fixed or Threshold directory we normally don't
scale it. However, in the case of HiDPI scaling we *do* want to
scale it, to avoid different layouts in Lo/HiDPI. We look up whatever
the size of the icon would have been in LoDPI and scale to that
in the no-scaling case, thus getting the same layout as the
unscaled case.

https://bugzilla.gnome.org/show_bug.cgi?id=708384