Occasional bad tiles on random polygons

@morganherlocker when feeding tile-cover with `require('geojson-random').polygon(10)`, it occasionally includes a bad tile that looks like this: `z: -24, x: -435.99999994039536, y: -1.9999999403953552`.

Looks like some backwards loop doesn't stop at 0 somewhere. It may be caused by polygons that exceed normally expected coords (-180..180, -85..85), but not sure.

This is the what caused [livetile](https://github.com/mapbox/livetile) builds to fail sporadically. cc @rclark

Start by reproducing the issue with require('geojson-random').polygon(10) as input to tile-cover, then inspect the geometry traversal and backward loops involved in producing tiles. Done means random polygons no longer produce tiles with negative zoom or coordinates such as z: -24, x: -435.99999994039536, y: -1.9999999403953552.

Written by the indexing model from the issue text.