need to handle multiple layers with the same name

[jfirebaugh]

If you composite two VTs with the same layer name, the resulting vector tile will have two layers with that name.

Background:

• https://gist.github.com/springmeyer/3522514c85443b57e338
• Handling duplicately named vtile source layers / Ability to filter features on source mapnik-vector-tile#20

Currently, vector-tile-js will drop all but the last encountered layer. Instead, it should either:

• merge features from all layers with the same name, or
• store layers in an array rather than a map

[springmeyer]

This is explicit in the vector tile spec now: A Vector Tile MUST NOT contain two or more layers whose name values are byte-for-byte identical. Prior to appending a layer to an existing Vector Tile, an encoder MUST check the existing name fields in order to prevent duplication. : https://github.com/mapbox/vector-tile-spec/blob/master/2.1/README.md#41-layers.

So, I think that vector-tile-js should expect non-duplicate names. And if duplicates are encountered throwing is valid. Agree @joto @mapsam?

Landing here in prep for https://github.com/mapbox/mvt-fixtures being brought into vector-tile-js. @mapsam do we have a duplicate layer name "invalid" fixture?

[joto]

Checking in the decoder that there are no duplicate names is expensive to do, so I think it should be allowed to assume that there are no duplicate names. But if it wants to check, it is allowed to do so and then can either fail or do something else.