[vector-api] Draw vector features in the order polygons, lines, images

This PR enforces a drawing order *within the same z-index* for different types. Lines are drawn on top of polygons and images are drawn on top of lines. This prevents polygons from obscuring lines and lines from obscuring images, which looks better. Compare and contrast:

Before:
![before](https://f.cloud.github.com/assets/6942/1858978/12a6f4a6-779a-11e3-8b4a-6ca4b1e85e6e.png)

After:
![after](https://f.cloud.github.com/assets/6942/1858982/1987e8ca-779a-11e3-9da4-8ae3e17e7680.png)