A big offset on polygon can show weird results

See screenshot (top and left border) using this STYLE:
```
	STYLE
	    COLOR 100 0 0
	    OUTLINECOLOR 0 0 80
            WIDTH 5
	END
      STYLE
         SYMBOL 'point_bleu'
         COLOR 200 10 10
         OUTLINECOLOR 10 10 200
         OFFSET 15 15
         SIZE 5.0
      END
```

![but_polygone_en_ligne_with_offset](https://f.cloud.github.com/assets/660191/33911/0337949c-50dd-11e2-833a-d32b400e777a.png)