[AUR-1829] Increase the size of the tile boundary

We can't rely on which direction TopoJSON will round in,
especially since the transform is applied on the client.

The solution is to move the tile boundaries slightly, by a
relative factor of the machine epsilon (I used the one for
32-bit floats). Any overlaps will be dealt with by the ST_Union
operation on the client.

Change-Id: Iac5bcaa842748dec8cd9c9fbf82540b0aa443927