thumbnailer: Bump chunk size to 65536
thumbnailer: Bump chunk size to 65536

As used in other parts of gdk-pixbuf.

https://bugzilla.gnome.org/show_bug.cgi?id=768062

GNOME/gdk-pixbuf,GNOME/gdk-pixbuf,GNOME/gdk-pixbuf,GNOME/gdk-pixbuf