Removed artifacts when draw text.

I found this issue on macbook pro with retina (mac os 10.10).
Before:
![before](https://cloud.githubusercontent.com/assets/3009258/5564289/4d2f5668-8ed7-11e4-8ed2-a29367affb06.png)
After:
![after](https://cloud.githubusercontent.com/assets/3009258/5564292/5f7c0be0-8ed7-11e4-8b41-3af43fcfb638.png)
See conversation on #66