Add a tile context to map pixel coordinates back to the world.
Fix rounding for the starting point to stay within the tile.
Add a raster (text) output format: a list of lat,lon pairs.

It's a lot like dumping, except that

  * The list of points is simplified for the specified zoom level,
    rather than complete for the tile
  * Lines are output as a list of points, not as start and endpoint.

The idea is that you can use it to compare how well two vector
sources match at a particular zoom level in a way that will still
render in a useful way at lower zooms.