forbit to use copy-object of link cascoords object

see  #94.

```
-	 (let ((l2 (copy-object l)))
-	   (send l2 :transform c :local)
-	   (send l2 :worldcoords)
-	   (flatten (send-all (send l2 :faces) :vertices))))
```
 caused memory leak.

thx @garaemon and @YoheiKakiuchi