indexing error in tile_datasource_pbf

[springmeyer]

While porting node-mapnik to use tile_datasource_pbf in https://github.com/mapnik/node-mapnik/issues/422 I hit:

  426 passing (4s)
  1 pending
  4 failing

  1) mapnik.VectorTile.composite should render with simple concatenation:
     Uncaught Error: vector

  2) mapnik.VectorTile.composite should render with image concatenation:
     Uncaught Error: vector

  3) mapnik.VectorTile.composite should render by overzooming:
     Uncaught Error: vector

  4) mapnik.VectorTile.composite should render by underzooming or mosaicing:
     Uncaught Error: vector

What is happening:

• A buffer is sent into tile_datasource_pbf and passed into the agg_renderer here: https://github.com/mapnik/node-mapnik/blob/0f839fcc36f2f3c75c20e50db30c6e1766f85e68/src/mapnik_vector_tile.cpp#L2952-L2961
• Then agg_renderer starts calling .features() and a featureset it created
• We then hit a situation where val_idx is > layer_values.size() here: https://github.com/mapbox/mapnik-vector-tile/blob/a480cb13a0b6881d42e24a19fc8d5d721df8c745/src/vector_tile_datasource_pbf.ipp#L113

[springmeyer]

ended up re-writing in #139