tile spec v2 support: prepareLayer defaults to version 1 which throws warning in mapbox-gl-js

[lukeelmers]

Right now prepareLayer defaults to version 1 if no version is provided. This causes mapbox-gl-js to throw a warning that the tile source does not use vector tile spec v2.

There doesn't seem to be a way to provide version in fromGeojsonVt that can override this default, but it would be awesome to have tile spec v2 support when converting from GeoJSON.

[gmaclennan]

@lukeelmers I believe a workaround for this would be to use GeoJSONWrapper which is exported but not documented, copying the code from https://github.com/mapbox/vt-pbf/blob/master/index.js#L33-L40

e.g. instead of:

var buff = vtpbf.fromGeojsonVt({ 'geojsonLayer': tile })

do:

var layer = new GeoJSONWrapper(tile.features)
layer.name = 'geojsonLayer'
layer.version = 2
var buff = vtpbf.fromVectorTileJs({layers: {
  geojsonLayer: layer
})

[redbmk]

@gmaclennan It looks like fromGeojsonVt now takes an options param, so you can set the version from there. e.g.

var buff = vtpbf.fromGeojsonVt({ 'geojsonLayer': tile }, { version: 2 })