Symbol placement - duplication

[d1manson]

Currently, this fork does not use the fancy symbol placement machinery from the upstream project. This is partly for performance and partly because it was complicated to try and work out how to make it work here.

One side effect seems to be that text is sometimes duplicated. I have a feeling this is happening because the text is on a polygon source layer, and is shown at the center of the polygons, but once per source tile that the polygon features on. A work around is to not use text on polygons, instead providing point data in your source tiles. It would however be nice to support it.