IDs ignored

[joto]

IDs < 0 and IDs not of type int64_t or uint64_t are simply ignored here:
https://github.com/mapbox/vector-tile/blob/2.x/include/mapbox/vector_tile/builder.hpp#L48-L59

Is this correct? If yes, I think it needs to be documented.

[flippmoke]

I think the core problem is more that we support int64_t and uint64_t id types in our internal geometry.hpp structure as well. We should probably look into only having one of these, but yes it should be documented.