[vector-api] Always quantize polygons

This fixes the bug described in #1408.

Before and after screenshots attached.
![screenshot_2013-12-17-17-41-38](https://f.cloud.github.com/assets/6942/1765884/ce1e5900-673b-11e3-9099-973720746db3.png)
![screenshot_2013-12-17-17-45-20 1](https://f.cloud.github.com/assets/6942/1765873/b17fd26a-673b-11e3-8846-27e466c54468.png)