Maps · Layers: routes, GeoJSON, heat and clusters

MapView takes layers over its tiles and under its pins. RouteLine runs
along places the short way across the date line. read_geojson reads RFC
7946 to features, strictly: a position past the globe, a line of one
position or a ring that does not close fails with its reason, and a
feature placed nowhere keeps no geometry. GeoJsonLayer washes areas as
one path per polygon, so holes stay open, outlines them, strokes lines
and dots points. GeoHeatmap spreads weighted places in a quartic kernel
over small cells, scaled to the densest spot in view. Lines and areas
unwrap across the date line and repeat on every world in view.
cluster_markers gathers pins by squares of the world, so a pan keeps
each group; a cluster shows its count, and its press shows its members
whole, never farther out. A zoom that may regroup them hands a focused
pin's or cluster's focus to the map.

src/canvas/mod.rs

@@ -52,6 +52,7 @@ pub use mindmap::Topic;
 pub use mindmap_view::MindMap;
 pub use minimap::MiniMap;
 pub use nodes::NodeGraph;
+pub(crate) use paint::{at, finish, outline};
 pub use palette::{BrushSettings, ToolPalette};
 pub(crate) use panel::{OnEdit, caption, color_well, editing};
 pub use plane::InfiniteCanvas;