render-icons.py: Merge in py3 fixes from Adwaita

render-icons.py was copied from gnome-icon-theme and changed slighly.
Since then gnome-icon-theme (now Adwaita) has done the job of making
the script python 3 compatible. This patch merges the required changes

Signed-off-by: Olof Kindgren <olof.kindgren@gmail.com>
Signed-off-by: Stef Walter <stefw@redhat.com>
 * Fixed some file paths

https://bugzilla.gnome.org/show_bug.cgi?id=739449