Fix bug #9456-#9485: Line Offset in symbology generates artifacts

This patch improves the offset of lines using GEOSOffsetCurve if GEOS 3.3 is available.

Bugs related:
http://hub.qgis.org/issues/9456
http://hub.qgis.org/issues/9485

Results:
![issue_9485-results](https://f.cloud.github.com/assets/5576272/2224697/34e941da-9a82-11e3-8523-d933c783668e.JPG)