gdk-pixbuf: add security patch for CVE-2015-4491

Fixes:
CVE-2015-4491 - Heap overflow in gdk-pixbuf when scaling bitmap images.
Patch from upstream:
https://git.gnome.org/browse/gdk-pixbuf/commit/?id=ffec86e



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>