Use a tile pyramid to speed up clipping
Same approach as in GeoGraphicsScene. Placemarks are assigned a tile level that fully contains them once. When clipping to some boundary box, only those placemarks on the path towards the top of the pyramid can intersect them. Note that this is only a heuristic; checking the actual bounding box intersection again afterwards still rules out a lot of placemarks that don't actually intersect. Rough speed comparison for different approaches: A) Slow: clip all placemarks directly B) Medium: clip all placemarks based on tile pyramid C) Fast: clip all placemarks that intersect with the bounding box D) Very fast: clip all placemarks based on tile pyramid that also intersect with the bounding box C) is the old implementation, B) something I tested locally and C) the new implementation.