Geometry shifts substantially between NE and OSM in boundaries layer

[nvkelso]

• What did you see?

Viewing the map at zoom 7 the shapes are over generalized from Natural Earth, and their alignment seems to be off. Viewing the map at zoom 8 from OSM the shapes are appropriately generalized for the zoom. Even if Natural Earth improved it's XY alignment issues, it would still be over generalized at zoom 7.

• What did you expect to see?

Adjust the NE to OSM transition from 8 to 7.

We did this once before in https://github.com/tilezen/vector-datasource/pull/1359/files, so "-1" to those PR values.

• Screenshot?

Zoom 7:
http://tangrams.github.io/bubble-wrap/#7.9/34.931/-282.051

Zoom 8:
http://tangrams.github.io/bubble-wrap/#8.1/34.8504/-282.5036