draw line by path to prevent thick line looks fractured

fix a bug while line is too thick:
![screenshot 2014-10-11 15 27 59](https://cloud.githubusercontent.com/assets/360744/4601851/0ad720de-511a-11e4-98b8-e91664715e82.png)

the line is not smooth, after fix:
![screenshot 2014-10-11 15 25 11](https://cloud.githubusercontent.com/assets/360744/4601852/184179f4-511a-11e4-934d-1e965becb894.png)