Use box2d<value_type> in decode_geometry

[springmeyer]

The bbox used in decode_geometry is currently hardcoded to double. It should be using the value_type for best performance:

mapnik-vector-tile/src/vector_tile_geometry_decoder.ipp

Lines 775 to 784 in 69d0d8e

	template <typename value_type>
	MAPNIK_VECTOR_INLINE mapnik::geometry::geometry<value_type> decode_geometry(GeometryPBF & paths,
	int32_t geom_type,
	unsigned version,
	value_type tile_x,
	value_type tile_y,
	double scale_x,
	double scale_y,
	mapnik::box2d<double> const& bbox)
	{

Waiting on mapnik/mapnik#3405