Inspired by CoverageUnion not quite working... it was very fast, but gave invalid results.
This code (still needs cleaned up!) avoids unioning many polygons by instead delaying polygonization until the final step.
This allows us to remove duplicates along tile boundaries before passing *everything* to a `Polygonizer`.

There is probably still scope for some optimizations (e.g. replacing `LineDissolver`) and a need to clean up the code and update the comments. But currently it shortens the time to trace ~8 million vertices to ~25 seconds on my laptop, rather than >> 100 seconds.

So committing this code now, lest I break it too badly...