optimize vtile.composite

/cc @yhahn - 256 for a buffer size is way to aggressive of a default for compositing. I think it is better to have the defaults lower here so that we prioritize performance. This may lead to a few more clipped labels: I think the proper solution to be able to keep an adequate buffer-size would be to be able to know, per layer, what original buffer-size was used. This idea is tracked in https://github.com/mapbox/mapnik-vector-tile/issues/34.