Switches polygon scanline back to non-tile coordinates

Per discussion, rolls polygon scanning back to using top-of-tile + original segment float coordinates.

Assumption is that when combined with https://github.com/mapbox/tile-cover/issues/40 the current gaps (that are always along an intersecting line) will be covered nicely.

Current gap examples:

![ex3](https://cloud.githubusercontent.com/assets/83384/4891759/7b9b0634-63ab-11e4-85bf-30a6e472afcd.png)
![ex2](https://cloud.githubusercontent.com/assets/83384/4891758/7b9af91e-63ab-11e4-93bd-e46351ca4b3a.png)
![ex1](https://cloud.githubusercontent.com/assets/83384/4891760/7b9d187a-63ab-11e4-9695-78b1aeb9c507.png)