Don't run elements entirely contained inside a tile through clipping

This avoids the expensive clipping process and results in slightly smaller
tiles (no synthetic id and the real id stored delta encoded rather than
as tag value string).

Most importantly though, it passes OSM data through unaltered, unlike our
current clipping code which can reorder vertices. While that's not such
a big deal for polygons, that can significantly alter the semantics of
directional lines. That remains to be fixed for the clipped cases though.