Ability to pre-select layers

[MateoV]

Add the ability to select only certain layers. We can skip unwanted layers before decoding to increase efficiency. Rather than decoding everything first and throwing away data we can predetermine and decode only what is needed. A good example is for Unity where a developer may only want building & street data.

[wilhelmberg]

Implemented.

New usage:

            VectorTile vt = new VectorTile(data);
            foreach (var layerName in vt.LayerNames())
            {
                VectorTileLayer layer = vt.GetLayer(layerName);
            }