Add parameter if and how tile data should be clipped

[wilhelmberg]

Almost all tiles contain data outside the actual tile extent (needed for labeling, joining features crossing tile boundaries, ...).

E.g. overzoomed z18 tile (red: extent of requested tile, gray: data in that tile):

Some clients may need the data to be cut exactly at the boundary or at a specified buffer width.

Pseudo-code:

public IGeometry GetGeom(int? buffer) {
  if(null == buffer) { return geom.AsIs; }
  return geom.Clip(buffer.Value);
}

[wilhelmberg]

Added in https://github.com/mapbox/vector-tile-cs/pull/21