RsvgPathBuilder: don't add a terminating moveto after closepath

When we used libart, before Cairo, it was necessary to add a moveto after a closepath, since
the starting and ending points of a closed path were supposed to coincide.

Cairo handles closed paths without needing the caller to manually move back to
the starting point.

Signed-off-by: Federico Mena Quintero <federico@gnome.org>