Fix vector tile layers (closes #6426)
Fix vector tile layers
(closes #6426)

After switching from XHR->fetch, we now parse the response separately,
so the `vtToGeoJSON` function already has an ArrayBuffer to work with.

kartta-labs/iD,morray/iD,digidem/iD,morray/iD,openstreetmap/iD,openstreetmap/iD,digidem/iD,kartta-labs/iD,kartta-labs/iD,openstreetmap/iD