Set incomingLayer.feature even when adding subsequent parts to a collection.

If the `style` parameter is a function, then that function is supposed to be passed the GeoJSON feature every time it is called. But if you create a layer with a `unique` function, so that features are grouped into collections, the `style` function only receives the feature the first time it is called for each collection, and styling of the subsequent parts added to the collection doesn't work. I believe this patch fixes this problem.